[SLPVectorizer] Begin abstracting InstructionsState alternate matching away from opcodes. NFCI.

This is an early step towards matching Instructions by attributes other than the opcode. This will be necessary for cast/call alternates which share the same opcode but have different types/intrinsicIDs etc. - which we could vectorize as long as we split them using the alternate mechanism.

Differential Revision: https://reviews.llvm.org/D48945

llvm-svn: 336344