[llvm] [AMDGPU][Attributor] Make `AAAMDFlatWorkGroupSize` honor existing attribute (PR #114357)

[Shilei Tian via llvm-commits]

shiltian wrote:

Can you disassemble the `.bc` to `.ll` and then check again? Directly checking `.bc` will not work because they are compressed/encoded. You can use `/soft/compilers/rocm/rocm-6.3.0/llvm/bin/llvm-dis xxx.bc -o xxx.ll`.

https://github.com/llvm/llvm-project/pull/114357