[RISCV] Allow conversion of fp divisions to fp multiplications by the reciprocal

If the divisor is repeated at least twice, we will convert the FDIVs to the
calculation of the reciprocal and FMULs.

We perform the transformation only under fast-math mode. FDIVs must have
'arcp' flag.

Differential Revision: https://reviews.llvm.org/D140024