[RISCV] Recurse on second operand of two operand shuffles (#79197)

This builds on bdc41106.

This change completes the migration to a recursive shuffle lowering
strategy where when we encounter an unknown two argument shuffle, we
lower each operand as a single source permute, and then use a vselect
(i.e. a vmerge) to combine the results. This relies for code quality on
the post-isel combine which will aggressively fold that vmerge back into
the materialization of the second operand if possible.

Note: The change includes only the most immediately obvious of the
stylistic cleanup. There's a bunch of code movement that this enables
that I'll do as a separate patch as rolling it into this creates an
unreadable diff.