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

[Quentin Dian via llvm-commits]

DianQK wrote:

> I am confused, the issue linked says you can remove memcpy, but the test that shows this only shows it being replaced with memmove?

`instcombine` or `dse` will remove `memcpy`: https://llvm.godbolt.org/z/EbxvWvrYb.
Of course, I could just delete it directly in `memcpyopt`, but that would make the code less charming. This is for `memcpy` forwarding.

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