[RISCV] Add the passthru operand for some RVV nomask unary and nullary intrinsics.

The goal is support tail and mask policy in RVV builtins.
We focus on IR part first.
If the passthru operand is undef, we use tail agnostic, otherwise
use tail undisturbed.

My plan is to handle more complex operations in follow-up patches.

Reviewers: frasercrmck

Differential Revision: https://reviews.llvm.org/D118253