[RISCV] Add initial SDNode patterns for unary zvbb instructions

This patch adds pseudos and SDNode patterns for vbrev.v, vrev8.v, vclz.v,
vctz.v and vcpop.v.
I've only added them for integer element types so far since we're lacking tests
for floats.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D155216