[RISCV] Add missed HasRoundModeOp for VPseudoUnaryMask_FRM/VPseudoUnaryMask_FRM.

Missed HasRoundModeOp makes performCombineVMergeAndVOps use wrong operands for
VFCVT_RM instructions.

Reviewed By: luke

Differential Revision: https://reviews.llvm.org/D158711