[RISCV] Remove exclusions for zext.h/zext.w from our (and (srl X, C1), C2) selection code.

This code tries to replace the pattern with a pair of shifts, but
we were excluding if the And could be a zext.h or zext.w. The SLLI/SRL
pair is more compressible and doesn't come with much down side.

We do regress one test case in rv64i-exhaustive-w-insts.ll but we
can probably add a narrower exclusion for that case.