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

[Quentin Dian via llvm-commits]

DianQK wrote:

> Can you add some tests to demonstrate that the staging allocas will be eliminated after memcpy forwarding?

Done. I've also updated some tests related to alignment.

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