[llvm] [MCP] Optimize copies when src is used during backward propagation (PR #111130)

[Vladimir Radosavljevic via llvm-commits]

vladimirradosavljevic wrote:

Rebased and updated checks in `midpoint-int.ll`.
@qcolombet Thanks for the review! Could you please merge this PR on my behalf?

https://github.com/llvm/llvm-project/pull/111130