[InstCombine] Extend peephole DSE to handle unordered atomics

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255932 91177308-0d34-0410-b5e6-96231b3b80d8