LICM: Don't sink stores out of loops that may throw.

Summary:
This hasn't been caught before because it requires noalias or similarly
strong alias analysis to actually reproduce.

Fixes http://llvm.org/PR27952 .

Reviewers: hfinkel, sanjoy

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D20944

llvm-svn: 271858