[MemCpyOpt] Check all access for MemoryUses in writtenBetween.

Currently writtenBetween can miss clobbers of Loc between End and Start,
if End is a MemoryUse.

To guarantee we see all write clobbers of Loc between Start and End
for MemoryUses, restrict to Start and End being in the same block
and check all accesses between them.

This fixes 2 mis-compiles illustrated in
llvm/test/Transforms/MemCpyOpt/memcpy-byval-forwarding-clobbers.ll

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D119929