[SelectionDAG][RISCV] Teach getConstant to use SPLAT_VECTOR_PARTS if vXi64...

[SelectionDAG][RISCV] Teach getConstant to use SPLAT_VECTOR_PARTS if vXi64 SPLAT_VECTOR is legal but i64 scalars are not.

That matches how such a SPLAT_VECTOR would have been type legalized
so assume it is ok to use for creating constants after type legalization.

Still need some improvements to SPLAT_VECTOR lowering.

This overlaps with some of what D158742 was trying to fix.

Reviewed By: luke

Differential Revision: https://reviews.llvm.org/D158870