[llvm] [SLP]Fix fmul/fadd fusion costs and retry FMA seeds after all blocks (PR #226117)

[Alexey Bataev via llvm-commits]

alexey-bataev wrote:

> > Someone should do it, I cannot cover everything myself
> 
> Can you share what you are covering, so we can understand where there might be gaps?

llvm-test-suite, spec2006/2017 for AVX512 at O3+LTO, + some other configs in som other corner cases

https://github.com/llvm/llvm-project/pull/226117