[llvm] [AMDGPU] Improve isBasicBlockPrologue helper function (PR #69924)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm requested changes to this pull request.

I think the title needs to be reworked. "Improved" doesn't really explain what this does 

https://github.com/llvm/llvm-project/pull/69924