[jit] stop parsing the block after seeing exit statements (#44870)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44870
fix https://github.com/pytorch/pytorch/issues/44864
Test Plan: buck test mode/dev-nosan //caffe2/test:jit -- 'test_assert_is_script'
Reviewed By: eellison
Differential Revision: D23755094
fbshipit-source-id: ca3f8b27dc6f9dc9364a22a1bce0e2f588ed4308
