[VE] Support convert instructions in MC layer

Summary:
Add CVTSQ/CVTDQ/CVTQD/CVTQS instructions.  Add regression tests for
them and other convert instructions of asmparser, mccodeemitter, and
disassembler.  In order to add those instructions, support RD operands
in asmparser, mccodeemitter, and disassembler.

Differential Revision: https://reviews.llvm.org/D81536