disable incorrect optimisation and enable tests for reversed iteration with negative step value

Cython/Compiler/Optimize.py

@@ -661,6 +661,9 @@ class IterationTransform(Visitor.EnvTransform):
         if reversed:
             bound1, bound2 = bound2, bound1
             if step_value < 0:
+                if step_value != -1:
+                    # FIXME: not currently supported
+                    return node
                 step_value = -step_value
             if step_value != 1:
                 begin_value = bound1.constant_result

tests/run/reversed_iteration.pyx

@@ -253,9 +253,9 @@ def reversed_range_constant():
 
 
 @cython.test_assert_path_exists('//ForFromStatNode')
-@cython.test_fail_if_path_exists('//ForInStatNode')
-def FIXME_reversed_range_constant_neg():
-    FIXME = """
+#@cython.test_fail_if_path_exists('//ForInStatNode')
+def reversed_range_constant_neg():
+    """
     >>> [ i for i in _reversed(range(-2, -12, -4)) ]
     [-10, -6, -2]
     >>> reversed_range_constant_neg()