[RISCV][POC] Model frm control for vfadd

Depends on D152879.

Specification PR: riscv-non-isa/rvv-intrinsic-doc#226

This patch adds variant of `vfadd` that models the rounding mode control.
The added variant has suffix `_rm` appended to differentiate from the
existing ones that does not alternate `frm` and uses whatever is inside.

The value `7` is used to indicate no rounding mode change. Reusing the
semantic from the rounding mode encoding for scalar floating-point
instructions.

Additional data member `HasFRMRoundModeOp` is added so we can append
`_rm` suffix for the fadd variants that models rounding mode control.

Additional data member `IsRVVFixedPoint` is added so we can define
pseudo instructions with rounding mode operand and distinguish the
instructions between fixed-point and floating-point.

Reviewed By: craig.topper, kito-cheng

Differential Revision: https://reviews.llvm.org/D152996