Fix/Implement CSE

compiler/src/l3/CPSOptimizer.scala

@@ -115,8 +115,7 @@ abstract class CPSOptimizer[T <: CPSTreeModule { type Name = Symbol }]
         val allLitOpt = replacedArgs.map(_.asLiteral)
         val isAllLit = allLitOpt.forall(_.isDefined)
         lazy val asLit = allLitOpt.map(_.get)
-        // TODO: fix this. For some reason it doesn't work
-        if (false && s.eInvEnv.isDefinedAt((prim, replacedArgs))) {
+        if (!unstable(prim) && !impure(prim) && s.eInvEnv.isDefinedAt((prim, replacedArgs))) {
           val preComputedAtom = s.eInvEnv((prim, replacedArgs))
           shrink(body, newState.withASubst(name, preComputedAtom))
         } else {