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

[Douglas Yung via llvm-commits]

dyung wrote:

@alexey-bataev we hit an assertion failure in one of our internal tests which I bisected back to this change. I have put the repro in #214181, can you take a look?

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