SLP Vectorizer: improve canonicalize tree operands of commutitive binary operands.

This reverts r214338 (except the test file) and replaces it with a more general algorithm.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@214485 91177308-0d34-0410-b5e6-96231b3b80d8