Tweak EarlyCSE to recognize series of dead stores

EarlyCSE is giving up on the current instruction immediately when it recognizes that the current instruction makes a previous store trivially dead. There's no reason to do this. Once the previous store has been deleted, it's perfectly legal to remember the value of the current store (for value forwarding) and the fact the store occurred (it could be dead too!).

Reviewed by: Hal
Differential Revision: http://reviews.llvm.org/D6301