[RISCV] Optimize splats of extracted vector elements

This patch adds an optimization to splat-like operations where the
splatted value is extracted from a identically-sized vector. On RVV we
can splat that via vrgather.vx/vrgather.vi without dropping to scalar
beforehand.

We do have a similar VECTOR_SHUFFLE-specific optimization but that only
works on fixed-length vector types and for those with a constant splat
lane. This patch extends this optimization to make it work on
scalable-vector types and on unknown extract indices.

It is performed during fixed-vector BUILD_VECTOR lowering and during a
new DAGCombine on SPLAT_VECTOR for scalable vectors.

Reviewed By: craig.topper, khchen

Differential Revision: https://reviews.llvm.org/D118456

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