[GVN] Don't replace constants with constants.

This fixes PR34908. Patch by Alex Crichton!

Differential Revision:  https://reviews.llvm.org/D38765

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315429 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed