[llvm] [SLP]Reduce number of alternate instruction, where possible (PR #128907)

[Alexey Bataev via llvm-commits]

alexey-bataev wrote:

> As this is a large patch, I was wondering if there is any way to disable it if something miscompiles?

Sure, I will add the option

https://github.com/llvm/llvm-project/pull/128907