[RISCV] Cleanup instruction formats used for B extension ternary operations.

Rename RVInstR4 as used by F/D/Zfh extensions to RVInstR4Frm.
Introduce new RVInstR4 that takes funct3 as a parameter.

Add new format classes for FSRI and FSRIW instead of trying to
bend RVInstR4 to use a shamt overlayed on rs2 and funct2.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D100427