[RISCV] Add subclasses of Sched to simplify code

We add some subclasses of Sched to reduce code which cast string
to `SchedWrite`/`SchedRead`.

Besides, we add `ReadVMask` iff pseudo is masked. And this is the
reason why this patch isn't a NFC since we add it for all pseudos
before.

`ReadMergeOp` will be added in D155654.

Reviewed By: michaelmaitland

Differential Revision: https://reviews.llvm.org/D155932