[llvm] [MCP] Handle iterative simplification during forward copy prop (PR #140267)

[Alex Bradbury via llvm-commits]

https://github.com/asb approved this pull request.

LGTM. I moderate that by saying that while I've attempted to follow the changes in MCP's logic and it makes sense, it's possible I miss something.

I'll note that this removes 547 instructions from an RVA22 llvm-test-suite build (with SPEC CPU2017).

https://github.com/llvm/llvm-project/pull/140267