bpo-33041: Rework compiling an "async for" loop. (#6142)

* Added new opcode END_ASYNC_FOR.
* Setting global StopAsyncIteration no longer breaks "async for" loops.
* Jumping into an "async for" loop is now disabled.
* Jumping out of an "async for" loop no longer corrupts the stack.
* Simplify the compiler.

Python/ceval.c

@@ -1944,6 +1944,26 @@ main_loop:
             }
         }
 
+        TARGET(END_ASYNC_FOR) {
+            PyObject *exc = POP();
+            assert(PyExceptionClass_Check(exc));
+            if (PyErr_GivenExceptionMatches(exc, PyExc_StopAsyncIteration)) {
+                PyTryBlock *b = PyFrame_BlockPop(f);
+                assert(b->b_type == EXCEPT_HANDLER);
+                Py_DECREF(exc);
+                UNWIND_EXCEPT_HANDLER(b);
+                Py_DECREF(POP());
+                JUMPBY(oparg);
+                FAST_DISPATCH();
+            }
+            else {
+                PyObject *val = POP();
+                PyObject *tb = POP();
+                PyErr_Restore(exc, val, tb);
+                goto exception_unwind;
+            }
+        }
+
         TARGET(LOAD_BUILD_CLASS) {
             _Py_IDENTIFIER(__build_class__);