[RISCV] Begin removing hasDummyMask.

This was used to know if we need to insert a dummy operand during
MCInstLowering. We can use the operand info from MCInstrDesc to
figure this out without needing a separate flag.

I'll remove the tablegen bits if there is consensus this is a good
idea.

Differential Revision: https://reviews.llvm.org/D152050