[SLPVectorizer] Merge reorderAltShuffleOperands into reorderInputsAccordingToOpcode

As discussed on D59738, this generalizes reorderInputsAccordingToOpcode to handle multiple + non-commutative instructions so we can get rid of reorderAltShuffleOperands and make use of the extra canonicalizations that reorderInputsAccordingToOpcode brings.

Differential Revision: https://reviews.llvm.org/D59784

llvm-svn: 356939