Summary:
I realized that control variable analysis was broken when we had multiple back-edges for the same loop. This is often the case when we have a switch statement combined with continue in a loop (see `test_switch` in `switch_continue.c`) or when we have disjunctive guards in do-while loops.
This diff fixes that by
- defining a loop by its loophead (the target of its backedges) rather than its back-edges. Then it converts back-edge list to a map from loop_head to sources of the loop's back-edges.
- collecting multiple guard nodes that come from potentially multiple exit nodes per loop head
In addition, it also removes the wrong assumption that an exit node belongs to a single loop head.
Reviewed By: mbouaziz
Differential Revision: D8398061
fbshipit-source-id: abaf288
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