Recommit "[SelectionDAG][RISCV] Add very basic PromoteIntegerResult/Op support...

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

I have fixed an existing DAGCombiner bug that caused the previous assertion failure.
See 71635394.

Original message

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