[llvm] [AMDGPU] Guard RewriteMFMAFormStage recolor against unsafe def/use (PR #217396)

[via llvm-commits]

xgxanq wrote:

> I think this needs to address the uses vs users question and a ready-for-merge patch to subregisters problem for this to go in. The former will help this be maintainable, the latter - resolve the regression this introduces (I understand this is just uncovered behavior by this patch, but it creates the problem nonetheless). The rest is nice-to-have and isn't blocking IMO.

Thanks very much.
1.What exactly does "address the uses vs users question" refer to? Could you show me the correct, suitable code for direct reference? Concepts alone are hard for me to fully grasp.
2.I’m personally fine with including the subreg-aware `findReachingDef` and use-related logic in this PR. That said, Matt previously advised that we should not combine multiple independent features within one patch, so I’m not sure which guidance to follow.

https://github.com/llvm/llvm-project/pull/217396