[flang] Internal error / crash in Preprocessor::IsIfPredicateTrue when evaluating macro expansion with operators

https://godbolt.org/z/qvdh5dqPd

The following code:

```fortran
#DEFINE WHICH == 2
#if WHICH == 1
```

Resulted in this output:
```
fatal internal error: no case at flang/lib/Parser/preprocessor.cpp(1212)
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: /usr/lib/llvm-22/bin/flang -fc1 -triple x86_64-pc-linux-gnu -emit-obj -mrelocation-model pic -pic-level 2 -pic-is-pie -target-cpu x86-64 -resource-dir /usr/lib/llvm-22/lib/clang/22 -mframe-pointer=all -o /tmp/test-f6b374.o -x f95 /tmp/test.f90
#0 0x00007f6c6e096d5f llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) (/usr/lib/llvm-22/lib/libLLVM.so.22.1+0x4dc7d5f)
#1 0x00007f6c6e0945d7 llvm::sys::RunSignalHandlers() (/usr/lib/llvm-22/lib/libLLVM.so.22.1+0x4dc55d7)
#2 0x00007f6c6e097b2a (/usr/lib/llvm-22/lib/libLLVM.so.22.1+0x4dc8b2a)
#3 0x00007f6c68d6b330 (/lib/x86_64-linux-gnu/libc.so.6+0x45330)
#4 0x00007f6c68dc4b2c pthread_kill (/lib/x86_64-linux-gnu/libc.so.6+0x9eb2c)
#5 0x00007f6c68d6b27e raise (/lib/x86_64-linux-gnu/libc.so.6+0x4527e)
#6 0x00007f6c68d4e8ff abort (/lib/x86_64-linux-gnu/libc.so.6+0x288ff)
#7 0x000056482167061c (/usr/lib/llvm-22/bin/flang+0x2b2061c)
#8 0x000056482134c79e (/usr/lib/llvm-22/bin/flang+0x27fc79e)
#9 0x000056482134941e Fortran::parser::Preprocessor::IsIfPredicateTrue(Fortran::parser::TokenSequence const&, unsigned long, unsigned long, Fortran::parser::Prescanner&) (/usr/lib/llvm-22/bin/flang+0x27f941e)
#10 0x0000564821346650 Fortran::parser::Preprocessor::Directive(Fortran::parser::TokenSequence const&, Fortran::parser::Prescanner&) (/usr/lib/llvm-22/bin/flang+0x27f6650)
#11 0x0000564821358d18 Fortran::parser::Prescanner::SkipCommentLine(bool) (/usr/lib/llvm-22/bin/flang+0x2808d18)
#12 0x0000564821359f29 Fortran::parser::Prescanner::FreeFormContinuation() (/usr/lib/llvm-22/bin/flang+0x2809f29)
#13 0x00005648213554a5 Fortran::parser::Prescanner::SkipToNextSignificantCharacter() (/usr/lib/llvm-22/bin/flang+0x28054a5)
#14 0x00005648213538e8 Fortran::parser::Prescanner::NextToken(Fortran::parser::TokenSequence&) (/usr/lib/llvm-22/bin/flang+0x28038e8)
#15 0x00005648213506eb Fortran::parser::Prescanner::Statement() (/usr/lib/llvm-22/bin/flang+0x28006eb)
#16 0x000056482134f5f8 Fortran::parser::Prescanner::Prescan(Fortran::common::Interval) (/usr/lib/llvm-22/bin/flang+0x27ff5f8)
#17 0x000056482133d052 Fortran::parser::Parsing::Prescan(std::__cxx11::basic_string, std::allocator> const&, Fortran::parser::Options) (/usr/lib/llvm-22/bin/flang+0x27ed052)
#18 0x000056481f71a732 Fortran::frontend::FrontendAction::runPrescan() (/usr/lib/llvm-22/bin/flang+0xbca732)
#19 0x000056481f71f7b5 Fortran::frontend::CodeGenAction::beginSourceFileAction() (/usr/lib/llvm-22/bin/flang+0xbcf7b5)
#20 0x000056481f719f9e Fortran::frontend::FrontendAction::beginSourceFile(Fortran::frontend::CompilerInstance&, Fortran::frontend::FrontendInputFile const&) (/usr/lib/llvm-22/bin/flang+0xbc9f9e)
#21 0x000056481f701e7f Fortran::frontend::CompilerInstance::executeAction(Fortran::frontend::FrontendAction&) (/usr/lib/llvm-22/bin/flang+0xbb1e7f)
#22 0x000056481f71eba0 Fortran::frontend::executeCompilerInvocation(Fortran::frontend::CompilerInstance*) (/usr/lib/llvm-22/bin/flang+0xbceba0)
#23 0x000056481f6ffd34 fc1_main(llvm::ArrayRef, char const*) (/usr/lib/llvm-22/bin/flang+0xbafd34)
#24 0x000056481f6fefa4 main (/usr/lib/llvm-22/bin/flang+0xbaefa4)
#25 0x00007f6c68d501ca (/lib/x86_64-linux-gnu/libc.so.6+0x2a1ca)
#26 0x00007f6c68d5028b __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28b)
#27 0x000056481f6fdf45 _start (/usr/lib/llvm-22/bin/flang+0xbadf45)
flang-22: error: unable to execute command: Aborted (core dumped)
flang-22: error: flang frontend command failed due to signal (use -v to see invocation)
Ubuntu flang version 22.1.8 (++20260613092238+e80beda6e255-1~exp1~20260613092253.78)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-22/bin
flang-22: error: unable to execute command: Aborted (core dumped)
flang-22: note: diagnostic msg: Error generating preprocessed source(s).
```

To reproduce:
```
flang ./test.f90
```

Compiler version:
```
Ubuntu flang version 22.1.8 (++20260613092238+e80beda6e255-1~exp1~20260613092253.78)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/lib/llvm-22/bin
```

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

Start with flang/lib/Parser/preprocessor.cpp around line 1212 and trace Preprocessor::IsIfPredicateTrue for the macro and operators in the issue. Run flang ./test.f90 using the supplied reproducer, then add or run a focused regression test showing the input is diagnosed or handled without an internal error or crash.

Written by the indexing model from the issue text.