[RISCV] Lower constant build_vectors with few non-sign bits via vsext (#65648)

If we have a build_vector such as [i64 0, i64 3, i64 1, i64 2], we
instead lower this as vsext([i8 0, i8 3, i8 1, i8 2]). For vectors with
4 or fewer elements, the resulting narrow vector can be generated via
scalar materialization.

For shuffles which get lowered to vrgathers, constant build_vectors of
small constants are idiomatic. As such, this change covers all shuffles
with an output type of 4 or less.

I deliberately started narrow here. I think it makes sense to expand
this to longer vectors, but we need a more robust profit model on the
recursive expansion. It's questionable if we want to do the zsext if
we're going to generate a constant pool load for the narrower type
anyways.

One possibility for future exploration is to allow the narrower VT to be
less than 8 bits. We can't use vsext for that, but we could use
something analogous to our widening interleave lowering with some extra
shifts and ands.

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