[VP][RISCV] Add vp.roundtozero and its RISC-V support

The scalar instruction of this is `llvm.trunc`. However the naming of
ISD::VP_TRUNC is already taken by `trunc` of the LLVM IR. Naming this as
`vp.ftrunc` would likely cause confusion with `vp.fptrunc`. So adding
`vp.roundtozero` that will look similar to `vp.roundeven`.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D135233