Fix &if crash after dch -f by leveling dangling choice nodes

[EPTansuo]

This PR fixes a reproducible assertion failure in Gia_ManChoiceLevel() when running &if on AIGs containing choices introduced by dch -f (reported in #466).

##Root Cause
dch -f can introduce choices that leave some AND nodes dangling (not reachable from CO/CI). Gia_ManChoiceLevel only computes levels by traversing CO/CI, so dangling ANDs keep level 0 and trip the assert.

Fix

Compute levels for any AND nodes that were not visited by the CO/CI traversal before the final assertion, so all ANDs have a valid level.

Test

abc -c "ra i10.aig; strash; dch -f; &get; &if -K 6;"

[wjrforcyber]

Emmm, the temporary fix LGTM but the reason doesn't look correct.

dch -f can introduce choices that leave some AND nodes dangling (not reachable from CO/CI). Gia_ManChoiceLevel only computes levels by traversing CO/CI, so dangling ANDs keep level 0 and trip the assert.

In the implementation of Gia_ManChoiceLevel_rec which is called byGia_ManChoiceLevel:

abc/src/aig/gia/giaIf.c

Lines 801 to 807 in 291e0a2

	// get the level of the nodes in the choice node
	if ( (pNext = Gia_ObjSiblObj(p, Gia_ObjId(p, pObj))) )
	{
	Gia_ManChoiceLevel_rec( p, pNext );
	if ( LevelMax < Gia_ObjLevel(p, pNext) )
	LevelMax = Gia_ObjLevel(p, pNext);
	}

These lines recursively calls the pNext on "siblings"(or "choice candidates", "dangling nodes") on original ANDs so all are calculated.
I don't have time recheck but I think it's something about &get since dch and &dch put choices in different structure, it has been modified several times, you could see that run a modified version of your flow

./abc -c "read_blif input.blif; strash; refactor; rewrite -z; &get; &dch -f; &ps; &if -K 4"

(which puts &dch in ABC9 space) everything's fine.