[Reassociation] Add support for reassociation with unsafe algebra.

Vector instructions are (still) not supported for either integer or floating
point.  Hopefully, that work will be landed shortly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215647 91177308-0d34-0410-b5e6-96231b3b80d8