[SelectionDAG] Remove duplicate declaration

Prefer the decl in SelectionDAGNodes.h because it's used there and
SelectionDAG.h includes SelectionDAGNodes.h.
[SelectionDAG] Pass DAG to checkForCycles

Pass the DAG down to checkForCycles from all callers where we have it.  This
allows target-specific nodes to be printed properly.

Also print some missing newlines.
[SelectionDAG] Force cycle detection in AssignTopologicalOrder before aborting

DAG cycle detection is only enabled with ENABLE_EXPENSIVE_CHECKS.  However we
can run it just before we would crash in order to provide more informative
diagnostics.

Now in addition to the "Overran sorted position" message we also get the Node
printed if a cycle was detected.

Tested by building several configs: Debug+Assert, Debug+Assert+Check (this is
ENABLE_EXPENSIVE_CHECKS), Release+Assert and Release.  Also tried that the
AssignTopologicalOrder assert produces the expected results.