Fix a query optimisation regression

Summary:
The recent changes exposed a regression with this query

```
0 where
  X1 = codemarkup.hack.HackFileEntityXRefLocations.2
    {file="www/RefClass.php"};
  codemarkup.hack.HackFileEntityXRefLocations.2
    {entity={decl={method=hack.MethodDeclaration.6 {}}}}=X1;
  X0=X1
```

This turns out to be pretty tricky to optimise. I'm adding yet more
flexibility to the final reordering pass that lets it pull out
lookups, which solves the problem in this case.

Reviewed By: iamirzhan

Differential Revision: D65059643

fbshipit-source-id: 879a869ea6d620b28c53454b084917fd0d8bfaa9

glean/db/Glean/Query/Reorder.hs

@@ -586,13 +586,17 @@ reorderStmts stmts = iterate stmts []
   choose _ [one] = ("only", (one, []))
   choose scope stmts = fromMaybe (error "choose") $
     firstResolved <|>
+    firstLookup <|>
     firstNotUnresolved <|>
     firstNotUnresolvedLenient <|>
     fallback
     where
       firstResolved = ("resolved",) <$>
         find (isResolvedFilter (ifBoundOnly scope) . fst) stmts'
 
+      firstLookup = ("lookup",) <$>
+        find (isLookup (ifBoundOnly scope) . fst) stmts'
+
       -- try to find a statement that's already resolved without
       -- adding any missing generators, and then try again allowing
       -- unbound predicates to be resolved.
@@ -665,6 +669,11 @@ isReadyFilter scope stmt notFilter = case stmt of
         appearInStmts
   _ -> notFilter
 
+isLookup :: InScope -> FlatStatement -> Bool
+isLookup scope (FlatStatement _ lhs FactGenerator{})
+  | Just (Var _ v _) <- isVar lhs, inScopeBound scope v = True
+isLookup _ _ = False
+
 data InScope = InScope
   { unboundPredicates :: Bool
   , isInScope :: Variable -> Bool