[RISCV] Use vslidedown for undef sub-sequences in generic build_vector

This is a follow up to D149263 which extends the generic vslide1down handling to use vslidedown (without the one) for undef elements, and in particular for undef sub-sequences. This both removes the domain crossing, and for undef subsequences results in fewer instructions over all.

Differential Revision: https://reviews.llvm.org/D149658#inline-1446673