[RISCV] Add an operand kind to the opcode/imm returned from RISCVMatInt.

Instead of matching opcodes to know the format to emit, use an
enum value that we can get from the RISCVMatInt::Inst class.

Change the consumers to use fully covered switches so that we get
a compiler warning if a new kind is added. With the opcode checks
it was easier to forget to update one of the 3 consumers.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D126317