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

[Alexey Bataev via llvm-commits]

alexey-bataev wrote:

> > 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).
> 
> 1. Main/alt opcode cannot be removed completely. X86 has instruction addsub, which effectively supports alt/main vectorization, plus too small alt/main combinations (<2) lead to inefficient vectorization.
> 2. Copyable elements vectorization will require some other stuff (possibly like adding pseudo-instructions for correct scheduling). This is the work I plan for the next patch.
> 
> > I guess I'm asking - is there a larger plan for this or is it focused on improving existing main-alt opcode performance?
> 
> Currently, this is mostly all we can do about main/alternate vectorization.

Also, as the first step to improve alt/main vectorization (and copyable elements), one of our team members works on https://github.com/llvm/llvm-project/pull/112181. It has some issues with compile time, we still investigating how we can improve it

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