[RISCV] Add i32 as a legal type for GPR register class.

I'm investigating if it is feasible to have i32 as a legal type for RV64.
The first thing we need to do is make i32 a valid type for the GPR
register class.

We already added f32/f64 as valid types which required adding explicit
types to tablegen patterns. Adding additional types to GPR is free now.

Reviewed By: sunshaoce

Differential Revision: https://reviews.llvm.org/D151177