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

[Christudasan Devadasan via llvm-commits]

cdevadas wrote:

> But I think you should have a test that would fail without this change (not the liverange-split related issue) and pass with this change?
I thought you were talking about the test case for liverange-split. I have a test for the SPILL issue. I can pre-commit the test first with the original issue indicated in it.


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