Fix SLPVectorizer commutativity reordering

The SLPVectorizer had a very crude way of trying to benefit
from associativity: it tried to optimize for splat/broadcast
or in order to have the same operator on the same side.
This is benefitial to the cost model and allows more vectorization
to occur.
This patch improve the logic and make the detection optimal (locally,
we don't look at the full tree but only at the immediate children).

Should fix https://llvm.org/bugs/show_bug.cgi?id=25247

Reviewers: mzolotukhin

Differential Revision: http://reviews.llvm.org/D13996

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 252337