[RISCV] Add a common base class for RVInstI variations. NFC

We have multiple variations of InstrFormatI that pack different
fields into the upper 12 bits. The other 20 bits are all the same.

Add base class to capture this commonality and allow subclasses to
explicitly define Inst{31-20}.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D156589