[MemCpyOpt] Merge alias metadatas when replacing arguments (#67539)

Alias metadata may no longer be valid after replacing the call argument.
Fix this by merging it with the memcpy alias metadata.

This fixes a miscompilation encountered in
https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/Failing.20tests.20when.20rustc.20is.20compiled.20with.201.20CGU.