[llvm] [RISCV] Added cost model for fmuladd (PR #125683)

[Philip Reames via llvm-commits]

https://github.com/preames requested changes to this pull request.

Revoking previous LGTM - per CI, change does not update all tests and fails to build.  Please fix.  

https://github.com/llvm/llvm-project/pull/125683