[llvm] [AMDGPU] Detect kills in register sets when trying to form V_CMPX instructions. (PR #68293)

[Jay Foad via llvm-commits]

jayfoad wrote:

> > > Can you just drop kill flags without prejudice? We shouldn't really need them anymore at this point
> > 
> > 
> > I am not sure about that.
> 
> In the general case we still use kill flags after this point in SIShrinkInstructions and SIPreEmitPeephole. More specifically can we drop kill flags from anything used by V_CMPX? I would say "yes", but I think that is basically what this patch is trying to do?
> 
> Perhaps Matt is suggesting that we permute all the sub and super registers of the registers which are passed to clearKillFlags, and call clearKillFlags on those as well.

I tried that in #68675.

https://github.com/llvm/llvm-project/pull/68293