[llvm] [SLP]Reduce number of alternate instruction, where possible (PR #123360)

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon commented:

I'm not clear whether this patch makes it easier or harder to remove the main-alt opcode kludge in the future and allow general splitting - copyable elements, more than 2 subnodes, non-matching instruction types (e.g. binop and intrinsic).

I guess I'm asking - is there a larger plan for this or is it focused on improving existing main-alt opcode performance?

https://github.com/llvm/llvm-project/pull/123360