[RISCV] Add an implementation of isFMAFasterThanFMulAndFAdd

[RISCV] Add an implementation of isFMAFasterThanFMulAndFAdd

Start with an assumption that FMA is faster than Fmul+FAdd. If thats not true
on some particular implementation we can add a tuning parameter in the future.

I've update the fmuladd test cases and added new test cases for fast math flag
based contraction.

Differential Revision: https://reviews.llvm.org/D91987