[LegalizeDAG] Return true from ExpandNode for some nodes that don't have expand support.

These nodes have a FIXME that they only get here because a Custom
handler returned SDValue() instead of the original Op.

Even though we aren't expanding them, we should return true here to
prevent ConvertNodeToLibcall from also trying to process them until
the FIXME has been addressed.

I'm hoping to add checking to ConvertNodeToLibcall to make sure
we don't give it nodes it doesn't have support for.