[RISCV] Optimize (sext.w, srli) to sraiw with Zba.

In this patch, we add a more narrower exclusion for
zeroext (srl x) -> srli (slli x), so that it provides an opportunity
for the selection of sraiw.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D120467