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

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xgxanq wrote:

> Can you just re-use the slightly simpler test case from #214124? No need for control flow to trigger the issue.

The IR shown in this card does not satisfy the cost check. Please also note that my modifications are not limited to the `rewrite()` function.

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