[Reassociate] Canonicalize negative constants out of expressions.

This is a reapplication of r221171, but we only perform the transformation
on expressions which include a multiplication.  We do not transform rem/div
operations as this doesn't appear to be safe in all cases.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221721 91177308-0d34-0410-b5e6-96231b3b80d8