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

[via llvm-commits]

bipmis wrote:

[mat-times-vec-slp-reproducer.tar.gz](https://github.com/user-attachments/files/32934481/mat-times-vec-slp-reproducer.tar.gz)
There is another regression scenario from llvm test suite - https://github.com/llvm/llvm-test-suite/blob/cde6a9c353752cc4050561e2b004f5d26e9e109f/Fortran/gfortran/regression/pr81303.f with a driver. The readme should have the steps to see the behaviour.

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