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

[via llvm-commits]

LWenH wrote:

Address @qcolombet 's comments, I add a default **CleanUp** parameter in the clobberRegister function to reuse this function. 

Although we might be able to do more **"CleanUp"** optimization when we calling the clobberRegister function. I think a more cautious solution currently might be only handle this case during the **ForwardCopyPropagateBlock** procedure as we encountering another COPY instruction with the same **Def**.

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