Remove a TODO related to llvm::isa<> .
Use the normal iteration pattern when dumping CFG nodes.
Add proper edge splitting at the CFG level.

This introduces a separate container pointing to IceCfgNodes that
represents the linearization / topological order, so that split nodes
can be inserted in the right place.  It also allows for a separate
re-linearization pass.

Edge splitting can be tested for now by enabling
IceCfgNode::multiblockCompensation().