Summary:
Release note: Use conditional returns to refine the path condition. Fixes#1182
When one branch of a conditional is an abrupt completion add the join condition to the path condition. The previous path condition is stored in the saved completion and restored at join point.
Closes https://github.com/facebook/prepack/pull/1212
Reviewed By: hermanventer
Differential Revision: D6562943
Pulled By: simonhj
fbshipit-source-id: fcf53a7c54118628bd3f62e5010f4c1046fd1145