[sledge] Add fast path to Context.partition_valid for empty existentials

Reviewed By: jvillard

Differential Revision: D25756556

fbshipit-source-id: 3bc85095d

sledge/src/fol/context.ml

@@ -199,7 +199,8 @@ end = struct
       in
       if s' != s then partition_valid_ t' ks' s' else (t', ks', s')
     in
-    partition_valid_ empty xs s
+    if Var.Set.is_empty xs then (s, Var.Set.empty, empty)
+    else partition_valid_ empty xs s
 
   (* direct representation manipulation *)