[llvm] [SLP]Flatten add/sub and fadd/fsub chains into one reassociated node (PR #213544)

[Alexey Bataev via llvm-commits]

alexey-bataev wrote:

> LGTM. For future work we can improve the flattening of the add/sub tree for cases when this is a mismatched number of negated and non-negated values.

I have some follow-up after this patch

https://github.com/llvm/llvm-project/pull/213544