llvm::DominatorTreeBase::recalculate called on empty function with if-converter

Repro

```llvm
define i32 @sext_known_nonzero(i16 %xx) {
%x = shl i16 256, %xx
%z = sext i16 %x to i32
%r = call i32 @llvm.cttz.i32(i32 %z, i1 false)
ret i32 %r
}
```

$ llc -start-before=if-converter test.ll

```
:DomTreeBuilder::SemiNCAInfo >::InfoRec; = void; reference = llvm::DomTreeBuilder::SemiNCAInfo >::InfoRec&; size_type = long unsigned int]: Assertion `idx < size()' 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: ./bin/llc a.ll -start-before=if-converter -print-after-all
1. Running pass 'Function Pass Manager' on module 'a.ll'.
2. Running pass 'X86 Assembly Printer' on function '@sext_known_nonzero'
Stack dump without symbol names (ensure you have llvm-symbolizer in your PATH or set the environment var `LLVM_SYMBOLIZER_PATH` to point to it):
0 llc 0x000055784892fdf6 llvm::sys::PrintStackTrace(llvm::raw_ostream&, int) + 59
1 llc 0x00005578489302ba
2 llc 0x000055784892d892 llvm::sys::RunSignalHandlers() + 131
3 llc 0x000055784892f75f
4 libc.so.6 0x00007f9077449df0
5 libc.so.6 0x00007f907749e95c
6 libc.so.6 0x00007f9077449cc2 gsignal + 18
7 libc.so.6 0x00007f90774324ac abort + 34
8 libc.so.6 0x00007f9077432420 __assert_perror_fail + 0
9 llc 0x000055784729b6f0
10 llc 0x0000557847297e83
11 llc 0x00005578472999a8
12 llc 0x00005578472953be
13 llc 0x00005578472918db
14 llc 0x000055784728fc3c void llvm::DomTreeBuilder::Calculate>(llvm::DominatorTreeBase&) + 29
15 llc 0x000055784728f922 llvm::DominatorTreeBase::recalculate(llvm::MachineFunction&) + 52
16 llc 0x0000557846e34e68 llvm::AsmPrinter::emitFunctionBody() + 292
17 llc 0x0000557845b5c481
18 llc 0x00005578472cdbbb llvm::MachineFunctionPass::runOnFunction(llvm::Function&) + 1117
19 llc 0x0000557847b5a06b llvm::FPPassManager::runOnFunction(llvm::Function&) + 639
20 llc 0x0000557847b5a2cd llvm::FPPassManager::runOnModule(llvm::Module&) + 97
21 llc 0x0000557847b5a6d8
22 llc 0x0000557847b55f22 llvm::legacy::PassManagerImpl::run(llvm::Module&) + 244
23 llc 0x0000557847b5af03 llvm::legacy::PassManager::run(llvm::Module&) + 39
```

Start by running the supplied test.ll reproducer with llc -start-before=if-converter and inspect the DominatorTreeBase::recalculate path shown in the stack trace. Follow how the if-converter leaves the function and how the X86 Assembly Printer reaches this path; done means the reproducer no longer triggers the assertion and the relevant compiler test covers it.

Written by the indexing model from the issue text.