[DSE] make sure memory is not modified before partial store merging (PR36129)

We missed a critical check in D30703. We must make sure that no intermediate 
store is sitting between the stores that we want to merge.

This should fix:
https://bugs.llvm.org/show_bug.cgi?id=36129

Differential Revision: https://reviews.llvm.org/D42663

llvm-svn: 323759