[RISCV][GISel] Make s32 a legal type for RV64 for any operation that has a W version.

My thought is that we can directly select W instructions using s32.

This will likely require combines and other optimizations eventually,
but this makes a simple starting point.

I'm slowly prototyping a similar approach for SelectionDAG.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D157770