[RISCV] : Add support for immediate operands.

llvm-exegesis uses operand type information provided in tablegen files to initialize
immediate arguments of the instruction. Some of them simply don't have such information.
Thus we should set into relevant immediate operands their specific type.
Also create verification methods for them.

Reviewed By: craig.topper
Differential Revision: https://reviews.llvm.org/D131771