[RISCV] Add more sign-extending ops to MIR sext.w pass.

This patch adds single-bit and bit-counting ops to list of sign-extending ops.

A single-bit write propagates sign-extendedness if it's not in the sign-bits.

Bit extraction and bit counting always outputs a small number, so sign-extended.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D121152