[RISCV] Separate hasRoundModeOpNum into separate VXRM and FRM functions.

Preparation for developing a new rounding mode insertion algorithm
that is going to be different between them since VXRM doesn't need
to be save/restored.

This also unifies the FRM handling in RISCVISelLowering.cpp between
scalar and vector.

Fixes outdated comments in RISCVAsmPrinter and sorts the predicate
function by the reverse order of the operands being skipped.

Reviewed By: eopXD

Differential Revision: https://reviews.llvm.org/D158326