[RISCV] Consider scalar types for required extensions.

We have vector operations on double vector and float scalar. For
example, vfwadd.wf is such a instruction.

vfloat64m1_t vfwadd_wf(vfloat64m1_t op0, float op1, size_t op2);

We should specify F and D extensions for it.

Differential Revision: https://reviews.llvm.org/D102051