[MemCpyOpt] keep src/dest alloca ordering (#176012)
Rather than test dominator of every use, just check which of src or dest
is first, and use that insert location. This minimizes unnecessary
dominator queries while also helping to preserve the order of allocas
(for better code readability / diff).
Extracted from PR optimization improvement series at
https://github.com/llvm/llvm-project/pull/150792
