[RISCV][GISel] Add isel patterns for ADDIW/SRLIW/SRAIW/SLLIW and remove custom selection. (#68470)

I had trouble getting patterns working previously because GISel was
using an i32 immediate, but the instructions expected an i64 immediate
because SelectionDAG doesn't have i32 as a legal type yet.

After looking at other targets like AMDGPU, I discovered that I could
use a SDNodeXForm and a cast to get the type checking in tablegen to
allow me to do it.