Make a pattern match more explicit.

This is looking at array dimensions in literals. By this point unknown
dimensions should have been inferred; if they haven't, I'd rather it blew up
here than caused bizarre behaviour later on.

(This is a good candidate for a property check.)

backends/BackendPasses.hs

@@ -219,7 +219,7 @@ declareSizesArray = doGeneric `ext1M` doStructured
     makeStaticSizeSpec :: Meta -> A.Name -> [A.Dimension] -> A.SpecType
     makeStaticSizeSpec m n ds = makeDynamicSizeSpec m n es
       where
-        es = [e | A.Dimension e <- ds]
+        es = [case d of A.Dimension e -> e | d <- ds]
 
     makeDynamicSizeSpec :: Meta -> A.Name -> [A.Expression] -> A.SpecType
     makeDynamicSizeSpec m n es = sizeSpecType