[GVNHoist] Fix non-deterministic sort order of PHIs for identical instructions

Summary: This fixes failure in Transforms/GVNHoist/hoist.ll uncovered by D39245.

Reviewers: hiraditya, spop, dberlin

Reviewed By: dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39410

llvm-svn: 316949