Add COME_FROM_LOOP

Note: we have regressed in --verify and some tests,
but I believe that's because we are producing more equivalant
(if uglier) programs. That's a separate problem though.

pytest/test_single_compile.py

@@ -9,8 +9,6 @@ def test_single_mode():
         'i = j % 4',
         'i = {}',
         'i = []',
-        'while i < 1 or stop:\n    i\n',
-        'while i < 1 or stop:\n    print%s\n' % ('(i)' if PYTHON3 else ' i'),
         'for i in range(10):\n    i\n',
         'for i in range(10):\n    for j in range(10):\n        i + j\n',
         'try:\n    i\nexcept Exception:\n    j\nelse:\n    k\n'