[RISCV] Remove codegen for vp_select (#194199)
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 vp.select from #179622
