Legalize EBBs in a reverse post-order.
Legalize EBBs in a reverse post-order.

This means that whenever we need to split a value, it is either already
defined by a concatenation instruction in a previously processed EBB, or
it's an EBB argument.

stoklund/cretonne,sunfishcode/cretonne,sunfishcode/cretonne,sunfishcode/cretonne,stoklund/cretonne,stoklund/cretonne