[SelectionDAG][RISCV] Add tests showing missed scalable-splat optimizations

These tests show missed opportunities in the SelectionDAG layer when
dealing with scalable-vector splats. All of these are handled for the
equivalent `ISD::BUILD_VECTOR` code, and the tests have largely been
translated from the equivalent X86 tests.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D106574