[flang] Assertion `mlir::isa<fir::BoxProcType>(handler.getType())' failed.

The following code:

```fortran
module m
contains
subroutine SETUP_SIGNALS_ffl(optional_status)
integer, parameter :: SIGFPE = 8
call signal(SIGFPE, HANDLER_ffl)
end subroutine
end module
```

Resulted in this output:
```
flang-24: /home/fuzz/WorkSpace/fusion-fuzz/projects/flang/llvm-project/flang/lib/Optimizer/Builder/Runtime/Intrinsics.cpp:434: void fir::runtime::genSignal(fir::FirOpBuilder &, mlir::Location, mlir::Value, mlir::Value, mlir::Value): Assertion `mlir::isa(handler.getType())' failed.
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace and instructions to reproduce the bug.
Stack dump:
0. Program arguments: /home/fuzz/WorkSpace/fusion-fuzz/projects/flang/llvm-flang-build/bin/flang-24 -fc1 -triple x86_64-unknown-linux-gnu -emit-obj -fno-color-diagnostics -mrelocation-model pic -pic-level 2 -pic-is-pie -target-cpu x86-64 -resource-dir /home/fuzz/WorkSpace/fusion-fuzz/projects/flang/llvm-flang-build/lib/clang/24 -mframe-pointer=all -o /tmp/min-f192c2.o -x f95 ./min.f90
#0 0x000055f63e82f33a ___interceptor_backtrace (/home/fuzz/WorkSpace/fusion-fuzz/projects/flang/llvm-flang-build/bin/flang-24+0x9f8933a)
#1 0x000055f640ca7ca3 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) /home/fuzz/WorkSpace/fusion-fuzz/projects/flang/llvm-project/llvm/lib/Support/Unix/Signals.inc:935:8
#2 0x000055f640ca1ae7 llvm::sys::RunSignalHandlers() /home/fuzz/WorkSpace/fusion-fuzz/projects/flang/llvm-project/llvm/lib/Support/Signals.cpp:0:5
#3 0x000055f640caa68f SignalHandler(int, siginfo_t*, void*) /home/fuzz/WorkSpace/fusion-fuzz/projects/flang/llvm-project/llvm/lib/Support/Unix/Signals.inc:474:38
#4 0x00007fe8398d2330 (/lib/x86_64-linux-gnu/libc.so.6+0x45330)
#5 0x00007fe83992bc0c pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x9ec0c)
#6 0x00007fe8398d227e raise (/lib/x86_64-linux-gnu/libc.so.6+0x4527e)
#7 0x00007fe8398b58ff abort (/lib/x86_64-linux-gnu/libc.so.6+0x288ff)
#8 0x00007fe8398b581b (/lib/x86_64-linux-gnu/libc.so.6+0x2881b)
#9 0x00007fe8398c8517 (/lib/x86_64-linux-gnu/libc.so.6+0x3b517)
#10 0x000055f64bd76926 fir::runtime::genSignal(fir::FirOpBuilder&, mlir::Location, mlir::Value, mlir::Value, mlir::Value) /home/fuzz/WorkSpace/fusion-fuzz/projects/flang/llvm-project/flang/lib/Optimizer/Builder/Runtime/Intrinsics.cpp:434:5
#11 0x000055f64bc87c98 Value /home/fuzz/WorkSpace/fusion-fuzz/projects/flang/llvm-project/flang/../mlir/include/mlir/IR/Value.h:98:56
#12 0x000055f64bc87c98 invokeHandler /home/fuzz/WorkSpace/fusion-fuzz/projects/flang/llvm-project/flang/lib/Optimizer/Builder/IntrinsicCall.cpp:2092:10
#13 0x000055f64bc87c98 operator())> /home/fuzz/WorkSpace/fusion-fuzz/projects/flang/llvm-project/flang/lib/Optimizer/Builder/IntrinsicCall.cpp:2121:16
#14 0x000055f64bc87c98 Log2VisitHelper<0UL, 3UL, fir::ExtendedValue, (lambda at /home/fuzz/WorkSpace/fusion-fuzz/projects/flang/llvm-project/flang/lib/Optimizer/Builder/IntrinsicCall.cpp:2120:7), const std::variant), fir::ExtendedValue (fir::IntrinsicLibrary::*)(mlir::Type, llvm::ArrayRef), void (fir::IntrinsicLibrary::*)(llvm::ArrayRef), fir::ExtendedValue (fir::IntrinsicLibrary::*)(std::optional, llvm::ArrayRef)> &> /home/fuzz/WorkSpace/fusion-fuzz/projects/flang/llvm-project/flang/include/flang/Common/visit.h:44:7
#15 0x000055f64bc87c98 visit<(lambda at /home/fuzz/WorkSpace/fusion-fuzz/projects/flang/llvm-project/flang/lib/Optimizer/Builder/IntrinsicCall.cpp:2120:7), const std::variant), fir::ExtendedValue (fir::IntrinsicLibrary::*)(mlir::Type, llvm::ArrayRef), void (fir::IntrinsicLibrary::*)(llvm::ArrayRef), fir::ExtendedValue (fir::IntrinsicLibrary::*)(std::optional, llvm::ArrayRef)> &> /home/fuzz/WorkSpace/fusion-fuzz/projects/flang/llvm-project/flang/include/flang/Common/visit.h:78:12
#16 0x000055f64bc87c98 fir::genIntrinsicCallHelper(fir::IntrinsicHandler const*, std::optional, llvm::ArrayRef, fir::IntrinsicLibrary&) /home/fuzz/WorkSpace/fusion-fuzz/projects/flang/llvm-project/flang/lib/Optimizer/Builder/IntrinsicCall.cpp:2119:17
#17 0x000055f64bbdb96a operator() /home/fuzz/WorkSpace/fusion-fuzz/projects/flang/llvm-project/flang/lib/Optimizer/Builder/IntrinsicCall.cpp:0:16
#18 0x000055f64bbdb96a __invoke_impl, (lambda at /home/fuzz/WorkSpace/fusion-fuzz/projects/flang/llvm-project/flang/lib/Optimizer/Builder/IntrinsicCall.cpp:2171:7), const fir::IntrinsicHandler *const &> /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/invoke.h:61:14
#19 0x000055f64bbdb96a __invoke<(lambda at /home/fuzz/WorkSpace/fusion-fuzz/projects/flang/llvm-project/flang/lib/Optimizer/Builder/IntrinsicCall.cpp:2171:7), const fir::IntrinsicHandler *const &> /usr/lib/gcc/x86_64-linux-gnu/13/../../../../include/c++/13/bits/invoke.h:96:14
```

Trunk output:
```
PLEASE submit a bug report to https://github.com/llvm/llvm-project/issues/ and include the crash backtrace and instructions to reproduce the bug.
#0 0x0000000004ae4a08 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/cefs/8a/8adaadf5797d75c89e042dd9_clang-llvmflang-trunk-20260904/bin/flang-24+0x4ae4a08)
#1 0x0000000004ae1e4d SignalHandler(int, siginfo_t*, void*) Signals.cpp:0:0
#2 0x000076f6c9a45330 (/lib/x86_64-linux-gnu/libc.so.6+0x45330)
#3 0x000000000d218473 mlir::Type::getContext() const (/cefs/8a/8adaadf5797d75c89e042dd9_clang-llvmflang-trunk-20260904/bin/flang-24+0xd218473)
#4 0x000000000d21e640 (anonymous namespace)::OperationVerifier::verifyOnEntrance(mlir::Operation&) Verifier.cpp:0:0
#5 0x000000000d2215eb (anonymous namespace)::OperationVerifier::verifyOperation(mlir::Operation&) Verifier.cpp:0:0
#6 0x000000000d22252a llvm::detail::UniqueFunctionBase::UniqueFunctionBase(mlir::MLIRContext*, mlir::Operation**, mlir::Operation**, (anonymous namespace)::OperationVerifier::verifyOnExit(mlir::Operation&)::'lambda'(mlir::Operation*)&&)::'lambda'(mlir::Operation**&&)>(mlir::MLIRContext*, mlir::Operation**, mlir::Operation**, (anonymous namespace)::OperationVerifier::verifyOnExit(mlir::Operation&)::'lambda'(mlir::Operation*)&&)::'lambda'(), llvm::LogicalResult mlir::failableParallelForEach(mlir::MLIRContext*, mlir::Operation**, mlir::Operation**, (anonymous namespace)::OperationVerifier::verifyOnExit(mlir::Operation&)::'lambda'(mlir::Operation*)&&)::'lambda'(mlir::Operation**&&)>(mlir::MLIRContext*, mlir::Operation**, mlir::Operation**, (anonymous namespace)::OperationVerifier::verifyOnExit(mlir::Operation&)::'lambda'(mlir::Operation*)&&)::'lambda'()>(mlir::Operation**, llvm::detail::UniqueFunctionBase::CalledAs<(anonymous namespace)::OperationVerifier::verifyOnExit(mlir::Operation&)::'lambda'(mlir::Operation*)>)::'lambda'(llvm::detail::UniqueFunctionBase const*)::_FUN(llvm::detail::UniqueFunctionBase const*) Verifier.cpp:0:0
#7 0x0000000004b56377 std::_Function_handler (), std::__future_base::_Task_setter, std::__future_base::_Result_base::_Deleter>, std::thread::_Invoker>>, void>>::_M_invoke(std::_Any_data const&) (/cefs/8a/8adaadf5797d75c89e042dd9_clang-llvmflang-trunk-20260904/bin/flang-24+0x4b56377)
#8 0x0000000004b56cbb std::__future_base::_State_baseV2::_M_do_set(std::function ()>*, bool*) (/cefs/8a/8adaadf5797d75c89e042dd9_clang-llvmflang-trunk-20260904/bin/flang-24+0x4b56cbb)
#9 0x000076f6c9aa1fb3 (/lib/x86_64-linux-gnu/libc.so.6+0xa1fb3)
#10 0x0000000004b569b1 std::__future_base::_Deferred_state>>, void>::_M_complete_async() (/cefs/8a/8adaadf5797d75c89e042dd9_clang-llvmflang-trunk-20260904/bin/flang-24+0x4b569b1)
#11 0x0000000004b5663c llvm::detail::UniqueFunctionBase::UniqueFunctionBase llvm::ThreadPoolInterface::asyncImpl(llvm::unique_function, llvm::ThreadPoolTaskGroup*)::'lambda'(), std::shared_future llvm::ThreadPoolInterface::asyncImpl(llvm::unique_function, llvm::ThreadPoolTaskGroup*)::'lambda'()>(void, llvm::detail::UniqueFunctionBase::CalledAs llvm::ThreadPoolInterface::asyncImpl(llvm::unique_function, llvm::ThreadPoolTaskGroup*)::'lambda'()>)::'lambda'(llvm::detail::UniqueFunctionBase const*)::_FUN(llvm::detail::UniqueFunctionBase const*) (/cefs/8a/8adaadf5797d75c89e042dd9_clang-llvmflang-trunk-20260904/bin/flang-24+0x4b5663c)
#12 0x000000000d294489 llvm::StdThreadPool::processTasks(llvm::ThreadPoolTaskGroup*) (/cefs/8a/8adaadf5797d75c89e042dd9_clang-llvmflang-trunk-20260904/bin/flang-24+0xd294489)
#13 0x000000000d294ac7 void* llvm::thread::ThreadProxy>(void*) ThreadPool.cpp:0:0
#14 0x000076f6c9a9cb84 (/lib/x86_64-linux-gnu/libc.so.6+0x9cb84)
#15 0x000076f6c9b29d6c (/lib/x86_64-linux-gnu/libc.so.6+0x129d6c)
flang-24: error: unable to execute command: Segmentation fault (core dumped)
flang-24: error: flang frontend command failed with exit code -2 (use -v to see invocation)
flang version 24.0.0git (https://github.com/llvm/llvm-project.git f6f14162f59d360509ad79ae96336223b724f44d)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /cefs/8a/8adaadf5797d75c89e042dd9_clang-llvmflang-trunk-20260904/bin
flang-24: note: diagnostic msg:
********************

PLEASE ATTACH THE FOLLOWING CRASH REPRODUCER FILES TO THE BUG REPORT:
flang-24: note: diagnostic msg: /tmp/example-8d4ca7
flang-24: note: diagnostic msg: /tmp/example-8d4ca7.sh
flang-24: note: diagnostic msg:

********************
flang-trunk - 277ms
```

Trunk: https://godbolt.org/z/6Eezb7v51

*This bug was found by [fusion-fuzz](https://github.com/fusion-fuzz/fusion-fuzz)*

Reproduce the crash with the Fortran module and signal call, then inspect flang/lib/Optimizer/Builder/Runtime/Intrinsics.cpp at genSignal and flang/lib/Optimizer/Builder/IntrinsicCall.cpp around invokeHandler. Trace why the handler is not a fir::BoxProcType and verify the compiler no longer asserts or segfaults on the provided reproducer.

Written by the indexing model from the issue text.