[Reassociate] Canonicalize negative constants out of expressions.

This gives CSE/GVN more options to eliminate duplicate expressions.
This is a follow up patch to http://reviews.llvm.org/D4904.

http://reviews.llvm.org/D5363

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221171 91177308-0d34-0410-b5e6-96231b3b80d8