[MemProf] Record accesses for all words touched in mem intrinsic

Previously for mem* intrinsics we only incremented the access count for
the first word in the range. However, after thinking it through I think
it makes more sense to record an access for every word in the range.
This better matches the behavior of inlined memory intrinsics, and also
allows better analysis of utilization at a future date.

Differential Revision: https://reviews.llvm.org/D110799