[RISCV] Use register class VR for V instruction operands directly.

@tangxingxin1008 found a bug that regard vadd.vv v1, v3, a0 as a valid V
instruction. We should remove the VRegAsmOperand operand class and use
VR register class directly.

Patched by: tangxingxin1008, Hsiangkai
Differential Revision: https://reviews.llvm.org/D91712