Explode store of arrays in instcombine

Summary: This is the last step toward supporting aggregate memory access in instcombine. This explodes stores of arrays into a serie of stores for each element, allowing them to be optimized.

Reviewers: joker.eph, reames, hfinkel, majnemer, mgrang

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D17828

llvm-svn: 262530