[RISCV] Use v(f)slide1down for build_vector with dominant values

If we have a dominant value, we can still use a v(f)slide1down to handle the last value in the vector if that value is neither undef nor the dominant value.

Note that we can extend this idea to any tail of elements, but that's ends up being a near complete merge of the v(f)slide1down insert path, and requires a bit more untangling on profitability heuristics first.

Differential Revision: https://reviews.llvm.org/D157120