[RISCV][SelectionDAG] Introduce an ISD::SPLAT_VECTOR_PARTS node that can...

[RISCV][SelectionDAG] Introduce an ISD::SPLAT_VECTOR_PARTS node that can represent a splat of 2 i32 values into a nxvXi64 vector for riscv32.

On riscv32, i64 isn't a legal scalar type but we would like to
support scalable vectors of i64.

This patch introduces a new node that can represent a splat made
of multiple scalar values. I've used this new node to solve the current
crashes we experience when getConstant is used after type legalization.

For RISCV, we are now default expanding SPLAT_VECTOR to SPLAT_VECTOR_PARTS
when needed and then handling the SPLAT_VECTOR_PARTS later during
LegalizeOps. I've remove the special case I previously put in for
ABS for D97991 as the default expansion is now able to succesfully
use getConstant.

Reviewed By: frasercrmck

Differential Revision: https://reviews.llvm.org/D98004