[RISCV] Add rvv codegen support for vp.fptrunc.

This patch adds rvv codegen support for vp.fptrunc. The lowering of fp_round and vp.fptrunc share most code so use a common lowering function to handle those two, similar to vp.trunc.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D123841