Implement load to store => memcpy in MemCpyOpt for aggregates

Summary:
Most of the tool chain is able to optimize scalar and memcpy like operation effisciently while it isn't that good with aggregates. In order to improve the support of aggregate, we try to change aggregate manipulation into either scalar or memcpy like ones whenever possible without loosing informations.

This is one such opportunity.

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

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D15894

llvm-svn: 256868