Fix two bugs causing problems with &trace and boxes.

[jon-greene]

Fix two bugs causing problems in &trace with boxes

Fork: jon-greene/abc
Branch: fix-req-time-with-boxes

Changed files:
src/aig/gia/giaSpeedup.c
src/aig/gia/giaAiger.c

BUG 1: Tim_ManGetCoRequired warning

Symptom:
When running &trace on designs with cascading boxes (two or more boxes
connected in series), the following warning is printed:
Tim_ManGetCoRequired(): Output required times of output 0 the box 1
are not up to date!

Cause:
In Gia_ManDelayTraceLut() (giaSpeedup.c), the box output CI level
correction loop reads the CO object level:

int coLevel = Gia_ObjLevel( p, Gia_ManCo(p, iCoFirst + j) );

But the CO object level was set by Gia_ManLevelNum before the CI level
corrections ran. For a box input CO whose fanin is a previous box's
output CI, the CO object level is stale (based on the CI's pre-correction
level of 0).

Example with two cascading boxes: Box 0's output CI w1 gets corrected
to level 3, but Box 1's input CO for w1 still has object level 0.
Box 1's output CI w2 then gets level 1 (maxLevel 0 + 1) instead of
level 4. In Gia_ManOrderReverse, w2 at level 1 is processed after
CO(w1->box1) at fanin level 3, causing Tim_ManGetCoRequired to find
that w2's required time has not been set yet.

Fix:
Read the CO's fanin level instead of the CO object level:

int coLevel = Gia_ObjLevel( p, Gia_ObjFanin0(Gia_ManCo(p, iCoFirst + j)) );

The fanin level reflects any prior CI corrections, so cascading boxes
pick up the corrected levels and their output CIs are placed at high
enough levels in Gia_ManOrderReverse.

BUG 2: XAIG ReqTime sentinel value

Symptom:
The output XAIG file contains ReqTime = 1000000000 (TIM_ETERNITY) for
unconstrained outputs. Per the XAIG spec, unconstrained required times
should be written as -1.0.

Cause:
In Gia_AigerWriteSimple() (giaAiger.c), Tim_ManGetReqTimes() returns
raw float values which may contain TIM_ETERNITY (1000000000). The code
writes these directly to the file via fwrite without converting them
to the XAIG spec's -1.0 sentinel.

Fix:
Write side (extension "o"): After Tim_ManGetReqTimes() and before
fwrite(), convert any values >= TIM_ETERNITY to -1.0.

Read side (extension "o"): After reading the required time array,
convert any values < 0 back to TIM_ETERNITY for internal use.