[InstCombine] Extend peephole DSE to handle unordered atomics

This extends the same line of reasoning used in EarlyCSE w/http://reviews.llvm.org/D15352 to the DSE implementation in InstCombine.

Key points:
 * We only remove unordered or simple stores.
 * The loads producing values consumed by dead stores don't influence whether the store is dead.

Differential Revision: http://reviews.llvm.org/D15354

llvm-svn: 255932
2 files changed