[RISCV] Fix illegal build_vector when lowering double id buildvec on RV32 (#67017)

When lowering a constant build_vector sequence of doubles on RV32, if
the
addend wasn't zero, or the step/denominator wasn't one, it would crash
trying
to emit an illegal build_vector of <n x i64> with i32 operands, e.g:

t15: v2i64 = BUILD_VECTOR Constant:i32<1>, Constant:i32<1>

This patch fixes this by lowering the splats with
SelectionDAG::getConstant
with the vector type, which handles making it legal via
splat_vector_parts.