[Attributor] Allow (inter-procedural) "CFG" reasoning for aligned regions

If an instruction is executed in an aligned region we can ignore
threading effects and use CFG reasoning (dominance and reachability).
This is true because all threads are together in an aligned region and
there cannot be one waiting for a signal at a place not connected via
the control flow.

More dedicated tests will follow.

More details can be found here:
"Co-Designing an OpenMP GPU Runtime and Optimizations for Near-Zero
Overhead Execution", IPDPS 2022,
https://www.osti.gov/servlets/purl/1890094