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

[Quentin Colombet via llvm-commits]

qcolombet wrote:

Hi @xgxanq,
For #1, we mean what Matt pointed out in https://github.com/llvm/llvm-project/pull/217396#discussion_r3956825782.
The use vs user concept is this:
```
def = op use1, use2
```
`use1` and `use2` are the uses, but the user is `op`. I.e., we want something that works at the instruction level, not the operand level. For instance if `use1` and `use2` are the same virtual registers, you would visit `op` twice with the use approach and only once with the user approach.

For #2, I think it depends whether the PR is useful in its current state. I don't have a personal opinion here.

Cheers,
-Quentin

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