Preserve nonnull metadata on Loads through SROA & mem2reg.

Summary:
https://llvm.org/bugs/show_bug.cgi?id=31142 :

SROA was dropping the nonnull metadata on loads from allocas that got optimized out. This patch simply preserves nonnull metadata on loads through SROA and mem2reg.

Reviewers: chandlerc, efriedma

Reviewed By: efriedma

Subscribers: hfinkel, spatel, efriedma, arielb1, davide, llvm-commits

Differential Revision: https://reviews.llvm.org/D27114

llvm-svn: 298540