[X86] canonicalizeShuffleMaskWithHorizOp - minor refactor to support multiple src ops. NFCI.

canonicalizeShuffleMaskWithHorizOp currently only supports shuffles with 1 or 2 sources, but PR41813 will require us to support higher numbers of sources.

This patch just generalizes the initial setup stages to ensure all src ops are the same type and opcode and then will continue to early out if we have more than 2 sources.