[SLPVectorizer] reorderInputsAccordingToOpcode - remove non-Instruction canonicalization

Remove attempts to commute non-Instructions to the LHS - the codegen changes appear to rely on chance more than anything else and also have a tendency to fight existing instcombine canonicalization which moves constants to the RHS of commutable binary ops.

This is prep work towards:
(a) reusing reorderInputsAccordingToOpcode for alt-shuffles and removing the similar reorderAltShuffleOperands
(b) improving reordering to optimized cases with commutable and non-commutable instructions to still find splat/consecutive ops.

Differential Revision: https://reviews.llvm.org/D59738

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@356913 91177308-0d34-0410-b5e6-96231b3b80d8
45 files changed