[llvm] (reland) [AMDGPU][SIInsertWaitCnts] Use RegUnits-based tracking (#162077) (PR #171779)

[Jay Foad via llvm-commits]

jayfoad wrote:

> Fixed a crash in Blender due to some weird control flow. The issue was with the "merge" function which was only looking at the keys of the "Other" VMem/SGPR maps. It needs to look at the keys of both maps and merge them.

I do not understand why this changes anything. The old code should have worked, because accessing `VMem[RegID].Scores[T]` implicitly created a default entry for `VMem[RegID]` if none existed before. I checked this when I reviewed the original patch!

> I did not add a new testcase, I want to but this issue is super difficult to reduce.

Do you even have an unreduced reproduser, where this fix makes any difference at all to the behaviour?

https://github.com/llvm/llvm-project/pull/171779