This patch adds more complex test cases with redundant stores of an
existing memset, with other stores in between.
It also makes a few of the existing tests more robust.
The MemorySSA-based implementation has been enabled without issue
for a while now, so keeping the old implementation around doesn't
seem useful anymore. This drops the MemDep-based implementation.
Differential Revision: https://reviews.llvm.org/D97877