Ignore certs when removing identity maps.

This can strictly speaking cause us to remove bounds checks in some
programs, but this implementation of flattening is going away soon
anyway.

src/Futhark/Pass/ExtractKernels/Distribution.hs

@@ -513,8 +513,8 @@ removeIdentityMappingGeneral bound pat res =
         expandTarget
       )
   where
-    isIdentity (patElem, SubExpRes cs (Var v))
-      | v `notNameIn` bound = Left (patElem, (cs, v))
+    isIdentity (patElem, SubExpRes _ (Var v))
+      | v `notNameIn` bound = Left (patElem, (mempty, v))
     isIdentity x = Right x
 
 removeIdentityMappingFromNesting ::