Update InstCombine to transform aggregate stores into scalar stores.

Summary: This is a first step toward getting proper support for aggregate loads and stores.

Test Plan: Added unittests

Reviewers: reames, chandlerc

Reviewed By: chandlerc

Subscribers: majnemer, joker.eph, chandlerc, llvm-commits

Differential Revision: http://reviews.llvm.org/D7780

Patch by Amaury Sechet

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 232284