Promote aggregate store to memset when possible

Summary: As per title. This will allow the optimizer to pick up on it.

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

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D15923

llvm-svn: 256969