Fix bug 25440: GVN assertion after coercing loads

Summary:
when coercing loads, it inserts some instructions, which have no GV assigned.

https://llvm.org/bugs/show_bug.cgi?id=25440


Reviewers: hfinkel, dberlin

Subscribers: dberlin, llvm-commits

Differential Revision: http://reviews.llvm.org/D14479

llvm-svn: 252919