[RISCV] Improve detection of when to skip (and (srl x, c2) c1) -> (srli (slli x, c3-c2), c3) isel.

We have a special case to skip this transform if c1 is 0xffffffff
and x is sext_inreg in order to use sraiw+zext.w. But we were only
checking that we have a sext_inreg opcode, not how many bits are
being sign extended.

This commit adds a check that it is a sext_inreg from i32 so we know for
sure that an sraiw can be created.