Clarify shape inference logic - commits to be refactored into one

aesara/tensor/subtensor.py

@@ -734,10 +734,20 @@ def make_node(self, x, *inputs):
                             continue
                 else:
                     if shp is not None:
+                        if p.start == 0 or p.start is None:
+                            start = 0
+                        else:
+                            start = p.start
+
+                        if p.stop == 0 or p.stop is None:
+                            stop = shp
+                        else:
+                            stop = p.stop
+
                         if p.step == 1 or p.step is None:
-                            length = max(p.stop - p.start, 0)
+                            length = max(stop - start, 0)
                         else:
-                            length = (p.stop - p.start - 1) // p.step + 1
+                            length = (stop - start - 1) // p.step + 1
                         static_shape.append(length)
                     else:
                         static_shape.append(None)