[DAGCombiner] Combine trunc (splat_vector x) -> splat_vector (trunc x)

From the discussion in https://reviews.llvm.org/D158853, moving the truncate
into the splat helps more splatted scalar operands get selected on RISC-V, and
also avoids the need for splat_vector_parts on RV32.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D159147