[RISCV] Use ComputeNumSignBits/MaskedValueIsZero in RISCVDAGToDAGISel::selectSExti32/selectZExti32.

This helps us select W instructions in more cases. Most of the
affected tests have had the sign_extend_inreg or AND folded into
sextload/zextload.

Differential Revision: https://reviews.llvm.org/D104079