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

[Quentin Dian via llvm-commits]

DianQK wrote:

@nikic https://llvm-compile-time-tracker.com/ is currently unavailable, possibly due to some meaningless force pushes I made that I apologize.

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