[RISCV][GISEL] Legalize G_BITCAST for scalable vectors (#85970)

SelectionDAG marks ISD::BITCAST as legal between scalable vector types
and ISelDAGToDAG deletes them.

We mark G_BITCAST between scalable vectors as legal in GISel. A future
patch will handle what to do with them after the legalizer (likley
either drop them in a isel-preprocess or convert them to COPYs).

BITCAST is needed for legalization of G_INSERT and G_EXTRACT. This is a
precommit for legalization of G_INSERT and G_EXTRACT.