C++: Avoid `iDominates*` in Overflow.qll
C++: Avoid `iDominates*` in Overflow.qll

The `iDominates` relation is directly on control-flow nodes, and its
transitive closure is far too large. It got compiled into a recursion
rather than `fastTC`, and I've observed that recursion to take about an
hour on a medium-size customer snapshot.

The fix is to check for dominance at the basic-block level.

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