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

[Petr Kurapov via llvm-commits]

kurapov-peter 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.

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