[SLP] Add support for commutative icmp/fcmp predicates

For the cases where the icmp/fcmp predicate is commutative, use reorderInputsAccordingToOpcode to collect and commute the operands.

This requires a helper to recognise commutativity in both general Instruction and CmpInstr types - the CmpInst::isCommutative doesn't overload the Instruction::isCommutative method for reasons I'm not clear on (maybe because its based on predicate not opcode?!?).

Differential Revision: https://reviews.llvm.org/D59992

llvm-svn: 357266