SLPVectorizer: Sort inputs to commutative binary operations

Sort the operands of the other entries in the current vectorization root
according to the first entry's operands opcodes.

%conv0 = uitofp ...
%load0 = load float ...

= fmul %conv0, %load0
= fmul %load0, %conv1
= fmul %load0, %conv2

Make sure that we recursively vectorize <%conv0, %conv1, %conv2> and <%load0,
%load0, %load0>.

This makes it more likely to obtain vectorizable trees. We have to be careful
when we sort that we don't destroy 'good' existing ordering implied by source
order.

radar://15080067