[SelectionDAG][RISCV] Add very basic PromoteIntegerResult/Op support for VP_SIGN/ZERO_EXTEND.

We don't have VP_ANY_EXTEND or VP_SIGN_EXTEND_INREG yet so I've
deviated a little from the non-VP lowering.

My goal was to fix the crashes that occurs on these test cases without this patch.

Reviewed By: fakepaper56

Differential Revision: https://reviews.llvm.org/D152854