[llvm] [SLP]Initial FMAD support (PR #149102)

[David Green via llvm-commits]

davemgreen wrote:

A note we saw some fallout from this in internal performance testing too. Something like this example doing a reduce of a fmul under fast-math is no longer vectorizing from the SLP vectorizer: https://godbolt.org/z/rYWM7dxEj. It wasn't helped on AArch64 by a different set of cost calculations that marked a fmul the same cost as a fma, but that example is x86. The original fadds in a reduction can be combined into a fma, but the expanded reduction will still become fma for part of it.

https://github.com/llvm/llvm-project/pull/149102