[DSE] Don't DSE stores that subsequent memmove calls read from

Summary:
We used to remove the first memmove in cases like this:

  memmove(p, p+2, 8);
  memmove(p, p+2, 8);

which is incorrect.  Fix this by changing isPossibleSelfRead to what was most
likely the intended behavior.

Historical note: the buggy code was added in https://reviews.llvm.org/rL120974
to address PR8728.

Reviewers: rsmith

Subscribers: mcrosier, llvm-commits, jlebar

Differential Revision: https://reviews.llvm.org/D43425

llvm-svn: 325641