NewGVN: Add basic dead and redundant store elimination

Summary:
This adds basic dead and redundant store elimination to
NewGVN.  Unlike our current DSE, it will happily do cross-block DSE if
it meets our requirements.

We get a bunch of DSE's simple.ll cases, and some stuff it doesn't.
Unlike DSE, however, we only try to eliminate stores of the same value
to the same memory location, not just general stores to the same
memory location.

Reviewers: davide

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29149

llvm-svn: 293258