[RISCV] Remove Opcode field from RVInst. Assign Inst{6-0} directly. NFC

Most places assign Opcode right after assigning every other bit in
Inst. I don't think treating Opcode separately adds much value. It
doesn't hide what bits belong to the opcode since every other bits is
listed.

This makes RVInst consistent with RVInst16 subclasss which already
assign Inst{1-0} directly.

Reviewed By: asb, wangpc

Differential Revision: https://reviews.llvm.org/D155797