[llvm] [MemCpyOpt] Calculate the offset value to forward `memcpy` (PR #87190)

[Quentin Dian via llvm-commits]

DianQK wrote:

@dtcxzyw Could you test this PR on llvm-opt-benchmark? This might be a common pattern in Rust.

https://github.com/llvm/llvm-project/pull/87190