[llvm] remove redundant waitcnts (PR #75785)

[Jay Foad via llvm-commits]

jayfoad wrote:

> > passes like GCNHazardRecognizer only recognize the non-soft forms.
> 
> Wouldn't this just be a hazard recognizer bug?

Well, that's a point of view. The distinction between the soft and normal forms of the instruction is by definition only relevant to SIInsertWaitcnts, so I thought it was simpler overall if later passes did not have to handle the soft forms.

https://github.com/llvm/llvm-project/pull/75785