[RISCV] Remove GetVRegNoV0 from the output register class of masked compare pseudo instructions.

Theses instructions are allowed to write v0 when they are masked.
We'll still never use v0 because of the earlyclobber constraint so
this doesn't really help anything. It just makes the definitions
correct.

While I was there remove an unused multiclass I noticed.

Reviewed By: HsiangKai

Differential Revision: https://reviews.llvm.org/D101118