RISC-V: Add rounding mode operand for fixed-point patterns
Since we are going to have fixed-point intrinsics that are modeling
rounding mode
https://github.com/riscv-non-isa/rvv-intrinsic-doc/pull/222
We should have operand to specify rounding mode in fixed-point instructions.
We don't support these modeling rounding mode intrinsics yet but we will
definetely support them later.
This is the preparing patch for new coming intrinsics.