[ValueTracking] Improve pointer offset computation for cases of same base

This patch improves the handling of pointer offset in GEP expressions where
one argument is the base pointer. isPointerOffset() is being used by memcpyopt
where current code synthesizes consecutive 32 bytes stores to one store and
two memset intrinsic calls. With this patch, we convert the stores to one
memset intrinsic.

Differential Revision: https://reviews.llvm.org/D67989

llvm-svn: 374454