Fix "for ... if" bug introduced since 3.6.2...

However we've isolated and documented the 3.x grammar a little bit better

test/simple_source/looping/10_for_if_loopback.py

@@ -11,6 +11,7 @@
 #       101_0	COME_FROM         '50' statement: if name == ...to fictional "end if"
 #         101	JUMP_BACK         '16' jump as before to loop head
 
+# RUNNABLE!
 def _slotnames(cls):
     names = []
     for c in cls.__mro__:
@@ -21,3 +22,18 @@ def _slotnames(cls):
                     continue
                 else:
                     names.append(name) # 3.2 bug here jumping to outer for
+
+# From 3.2.6 pdb.py
+# There is no "come_from" after the "if" since the
+# if jumps back to the loop. For this we use
+# grammar rule "ifstmtl"
+def lasti2lineno(linestarts, a):
+    for i in linestarts:
+        if a:
+            return a
+    return -1
+
+assert lasti2lineno([], True) == -1
+assert lasti2lineno([], False) == -1
+assert lasti2lineno([1], False) == -1
+assert lasti2lineno([1], True) == 1