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

[Matt Arsenault via llvm-commits]

arsenm wrote:

I think isBasicBlockPrologue is unworkable and should be removed. I don't think it's sensible to do ad-hoc exec mask tracking like this and we need a better control flow representation. I think for the moment we are better off splitting blocks, and then starting to augment MachineBasicBlock with an annotation of the divergent CFG. 

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