[RISCV] Share tablegen classes for F, D, and Zfh. Other simplifications. NFC

By adding the register class and funct as template parameters we
can share the classes with all 3 extensions.

I've used "let SchedRW =" to avoid repeating scheduler classes on
multiple lines where we previously inherited from the Sched class.

A subsequent patch will add mayRaiseFPException and FRM dependencies.
Reducing the number of classes means less repeating for those changes.

This of course conflicts with the Zfinx patch D93298.

Reviewed By: achieveartificialintelligence

Differential Revision: https://reviews.llvm.org/D115469