[llvm] [SPIRV] Fix warning in SPIRVMergeRegionExitTargets.cpp (PR #95283)

[Helena Kotas via llvm-commits]

hekota wrote:

I've seen it in my local build, this is my config:

`cmake -G "Ninja"  -DCMAKE_C_COMPILER=clang-cl -DCMAKE_CXX_COMPILER=clang-cl -DCMAKE_LINKER=lld-link -DCMAKE_BUILD_TYPE=Debug  -DLLVM_PARALLEL_COMPILE_JOBS=60 -DLLVM_PARALLEL_LINK_JOBS=4 -DCMAKE_WARN_DEPRECATED=FALSE -S C:\llvm-project\llvm -B C:\llvm-project\build\x64-Debug -C C:\llvm-project\clang\cmake\caches\HLSL.cmake`

https://github.com/llvm/llvm-project/pull/95283