[llvm] [SLP]Represent SLP graph as a tree (PR #126771)

[Alexey Bataev via llvm-commits]

alexey-bataev wrote:

> > CINT2017rate/525.x264_r - small variation + improvements in reordering, @pixel_hadamard_ac stopped
> > being vectorized because of some non-effective shuffle recognition by
> > the compiler
> 
> I think this might fix the regression in #124499?

I hope so, especially being combined with https://github.com/llvm/llvm-project/pull/123360, which I postponed before landing this patch

https://github.com/llvm/llvm-project/pull/126771