[llvm] [SimplifyCFG] Select the first instruction that we can handle in `passingValueIsAlwaysUndefined` (PR #98802)

[via llvm-commits]

DianQK wrote:

I can't reproduce the problem yet, can you provide a more detailed test step? 
My local test commands are:

```bash
# CMake project
clang++     -D_FILE_OFFSET_BITS=64    -DCMAKE_BOOTSTRAP    -DCMake_HAVE_CXX_MAKE_UNIQUE=1 -DCMake_HAVE_CXX_FILESYSTEM=1   -I/home/dianqk/llvm/CMake/build/Bootstrap.cmk   -I/home/dianqk/llvm/CMake/Source   -I/home/dianqk/llvm/CMake/Source/LexerParser   -I/home/dianqk/llvm/CMake/Utilities/std   -I/home/dianqk/llvm/CMake/Utilities  -c /home/dianqk/llvm/CMake/Source/cmGeneratorExpressionLexer.cxx -o cmGeneratorExpressionLexer.o -emit-llvm -O3 -Xclang -disable-llvm-passes

# cmake (stage 1)
cmake -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_ASSERTIONS=OFF -DBUILD_SHARED_LIBS=ON ...
# main branch
perf stat $LLVM_PROJECT/build-base/bin/opt -O2 cmGeneratorExpressionLexer.o --disable-output
339,409,679      instructions:u
# pr
perf stat $LLVM_PROJECT/build/bin/opt -O2 cmGeneratorExpressionLexer.o --disable-output
338,379,357      instructions:u
```

https://github.com/llvm/llvm-project/pull/98802