[RISCV] Support named opcodes in .insn directive.

This patch is one of the TODO of commit, 28387979

We build the GenericTable for these opcodes, and also extend class RISCVOpcode, to store the names of opcodes.  Then we call the parseInsnDirectiveOpcode to parse the opcode filed in .insn directive.  We only allow users to write the recognized opcode names, or just write the immediate values in the 7 bits range.

Documentation: https://sourceware.org/binutils/docs-2.37/as/RISC_002dV_002dFormats.html

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D115224