Fix load alignement when unpacking aggregates structs

Summary: Store and loads unpacked by instcombine do not always have the right alignement. This explicitely compute the alignement and set it.

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

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D17326

llvm-svn: 261139