[DSE] Remove noop stores after killing stores for a MemoryDef.

Currently we fail to eliminate some noop stores if there is a kill-able
store between the starting def and the load. This is because we
eliminate noop stores first.

In practice it seems like eliminating noop stores after the main
elimination for a def covers slightly more cases.

This patch improves the number of stores slightly in 2 cases for X86 -O3
-flto

Same hash: 235 (filtered out)
Remaining: 2
Metric: dse.NumRedundantStores

Program                                          base      patch diff
 test-suite...ce/Benchmarks/PAQ8p/paq8p.test     2.00   3.00 50.0%
 test-suite...006/453.povray/453.povray.test    18.00  21.00 16.7%

There might be other phase ordering issues, but it appears that they do
not show up in the test-suite/SPEC2000/SPEC2006. We can always tune the
ordering later.

Partly fixes PR47887.

Reviewed By: asbirlea, zoecarver

Differential Revision: https://reviews.llvm.org/D89650