[RISCV] Remove codegen for vp_zext, vp_sext (#194295)
Part of the work to remove trivial VP intrinsics from the RISC-V
backend, see
https://discourse.llvm.org/t/rfc-remove-codegen-support-for-trivial-vp-intrinsics-in-the-risc-v-backend/87999
This splits off 2 intrinsics from #179622. The vp_truncate combine needs
to be updated to keep the vaaddu patterns for now, but it will be
removed in an upcoming PR.
