[MemCpy] Check for alias in performMemCpyToMemSetOptzn, instead of the identity of two operands

Summary:
This fixes pr29105. The reason is that lifetime marks creates new
aliasing pointers the original ones, but before this patch aliases
were not checked in performMemCpyToMemSetOptzn.

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D23846

llvm-svn: 279769