Improve load/store to memcpy for aggregate

Summary: It turns out that if we don't try to do it at the store location, we can do it before any operation that alias the load, as long as no operation alias the store.

Reviewers: craig.topper, spatel, dexonsmith, Prazek, chandlerc, joker.eph

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D15903

llvm-svn: 256923