Update InstCombine to transform aggregate loads into scalar loads.

Summary:
One step further getting aggregate loads and store being optimized
properly. This will only handle struct with one element at this point.

Test Plan: Added unit tests for the new supported cases.

Reviewers: chandlerc, joker-eph, joker.eph, majnemer

Reviewed By: majnemer

Subscribers: pete, llvm-commits

Differential Revision: http://reviews.llvm.org/D8339

Patch by Amaury Sechet.

From: Amaury Sechet <amaury@fb.com>
llvm-svn: 236695