[llvm] [MCP] Enhance MCP copy Instruction removal for special case (PR #70778)

[Kai Luo via llvm-commits]

bzEq wrote:

> Shouldn't we forget about L1 when we get to L2?

I agree with we should forget about L1 when we get to L2. The semantic of `Copies[Unit].DefRegs` should be Registers defined by Unit in COPY instructions, we want to keep this invariant during tracking. I think it's appropriate to keep this property during `trackCopy`. After L2, `Copies[r9].DefRegs` still containing `r0` is violating this property.

https://github.com/llvm/llvm-project/pull/70778