compiler: Avoid nonsensical hoisting out of VirtualDimensions

Author: FabioLuporini

devito/passes/clusters/aliases.py

@@ -306,9 +306,13 @@ def process(self, clusters):
     def callback(self, clusters, prefix, xtracted=None):
         if not prefix:
             return clusters
+
         p = prefix[-1]
         d = p.dim
 
+        if d.is_Virtual:
+            return clusters
+
         # Rule out extractions that would break data dependencies
         exclude = set().union(*[c.scope.writes for c in clusters])