[EarlyCSE] DSE of stores which write back loaded values

Extend EarlyCSE with an additional style of dead store elimination. If we write back a value just read from that memory location, we can eliminate the store under the assumption that the value hasn't changed.

I'm implementing this mostly because I noticed the omission when looking at the code. It seemed strange to have InstCombine have a peephole which was more powerful than EarlyCSE. :)

Differential Revision: http://reviews.llvm.org/D15397

llvm-svn: 255739