[RISCV] Recursively split concat_vector into smaller LMULs when lowering (#85825)

This is a reimplementation of the combine added in #83035 but as a
lowering instead of a combine, so we don't regress the test case added
in e59f120e by interfering with the
strided load combine

Previously the combine had to concatenate the split vectors with
insert_subvector instead of concat_vectors to prevent an infinite
combine loop. And the reasoning behind keeping it as a combine was
because if we emitted the insert_subvector during lowering then we
didn't fold away inserts of undef subvectors.

However it turns out we can avoid this if we just do this in lowering
and select a concat_vector directly, since we get the undef folding for
free with `DAG.getNode(ISD::CONCAT_VECTOR, ...)` via foldCONCAT_VECTORS.