[RISCV] Correct the setcc in vp.floor/ceil/round/roundeven lowering.

We want to emit a masked setcc that preserves zeros in all of the bits
where the original mask is zero. To do this we need to pass the original
mask as the passthru operand as well. Otherwise, we'll use the mask agnostic
policy and replace the zeros with 1s on some CPUs.

Differential Revision: https://reviews.llvm.org/D135122