[RISCV] Improve legalization of e8 m8 VL>256 shuffles (#79330)

If we can't produce a large enough index vector in i8, we may need to legalize
the shuffle (via scalarization - which in turn gets lowered into stack usage).
This change makes two related changes:
* Deferring legalization until we actually need to generate the vrgather
  instruction.  With the new recursive structure, this only happens when
  doing the fallback for one of the arms.
* Check the actual mask values for something outside of the representable
  range.

Both are covered by recently added tests.