[Simplify] Remove writes that are overwritten.

Remove memory writes that are overwritten by later writes. This works
for StoreInsts:

      store double 21.0, double* %A
      store double 42.0, double* %A

scalar writes at the end of a statement and mixes of these.

Multiple writes can be the result of DeLICM, which might map multiple
writes to the same location when it knows that these do no conflict
(for instance because they write the same value). Such writes
interfere with pattern-matched optimization such as gemm and may not
get removed by other LLVM passes after code generation.

Differential Revision: https://reviews.llvm.org/D33142

llvm-svn: 302986