[sledge] Change: Normalize trivial equalities

Summary:
`Sh.norm` relies on `Eq (v, v)` formulas for `v` a variable to be
normalized to `Tt`, which is how eliminated variables are actually
removed from the representation.

Reviewed By: jvillard

Differential Revision: D22571132

fbshipit-source-id: 6d5f3efd7

sledge/src/fol.ml

@@ -197,7 +197,7 @@ end = struct
 
   let _Tt = Tt
   let _Ff = Ff
-  let _Eq x y = Eq (x, y)
+  let _Eq x y = if equal_trm x y then Tt else Eq (x, y)
   let _Dq x y = Dq (x, y)
   let _Eq0 x = Eq0 x