[MemCpyOpt] Fix up debug loc for simplified memset in processMemSetMemCpyDependence

Make sure the code comments in processMemSetMemCpyDependence match
with the actual transform. They indicated that the memset being
rewritten was sunk to after a memcpy, while it actually is inserted
just before the memcpy.

Also make sure we use the debug location of the original memset
when creating the new simplified memset. In the past we've been
using the debug location for the memcpy which could be a bit
confusing.

Differential Revision: https://reviews.llvm.org/D135574