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

We have multiple variations of InstrFormatR that pack different
fields into the upper 7 bits. The other 25 bits are all the same.

Add base class to capture this commonality and allow subclasses to
explicitly define Inst{31-25}.

Reviewed By: asb

Differential Revision: https://reviews.llvm.org/D156590