Avoid pointer chasing for pFanout->Id when we already have it in Aig_ManUpdateReverseLevel

[calewis]

The diff makes it hard to see, but doing some manual inlining shows the changes we are trying to make look like:
Before:

    pFanout = p->vObjs->pArray[iFanout >> 1];
    int FanoutId = pFanout->Id;
    Vec_IntFillExtra( p->vLevelR, FanoutId + 1, 0 );
    LevelCur = p->vLevelR->pArray[FanoutId];

After:

    int FanoutId = iFanout >> 1;
    Vec_IntFillExtra( p->vLevelR, FanoutId + 1, 0 );
    LevelCur = p->vLevelR->pArray[FanoutId];

By using Vec_IntGetEntry(p->vLevelR, FanoutId) directly, the memory loads to retrieve pFanout and dereference pFanout->Id are completely eliminated.

The Vec_IntFillExtra( p->vLevelR, Aig_ManObjNumMax(p), 0 ); seeks to avoid any resizing in the loop.

This can speed up Aig_ManUpdateReverseLevel about 40% in our tests, our profiling showed a large amount of time spent waiting on the pFanout data to fetch from memory.

There is an assumption that int FanoutId = iFanout >> 1; will remain valid in the future, but who ever is reviewing hopefully can comment on that.