use bdd_restrict more

Doing so reduced the number of GC passes tested in bdd.test, which is
good.

* spot/twaalgos/ltl2tgba_fm.cc: Simplify minato loops with
bdd_restrict.
* spot/twaalgos/synthesis.cc (split_2step): Use bdd_restrict instead
of bdd_appex.
* tests/core/bdd.test, tests/core/ltlf.test: Adjust test cases.

tests/core/ltlf.test

@@ -57,7 +57,7 @@ State: 3
 HOA: v1
 name: "a & X(A & a) & (A U G!A)"
 States: 4
-Start: 2
+Start: 3
 AP: 2 "A" "a"
 acc-name: Buchi
 Acceptance: 1 Inf(0)
@@ -70,9 +70,9 @@ State: 1
 [!0] 0
 [0] 1
 State: 2
-[0&1] 3
-State: 3
 [0&1] 1
+State: 3
+[0&1] 2
 --END--
 HOA: v1
 name: "(a U (A & b)) & (A U G!A) & F((A & c) | (A & d & X!A))"
@@ -124,7 +124,7 @@ State: 2 {0}
 --END--
 HOA: v1
 States: 3
-Start: 1
+Start: 2
 AP: 1 "a"
 acc-name: Buchi
 Acceptance: 1 Inf(0)
@@ -133,9 +133,9 @@ properties: trans-labels explicit-labels state-acc deterministic
 State: 0 {0}
 [t] 0
 State: 1
-[0] 2
-State: 2
 [0] 0
+State: 2
+[0] 1
 --END--
 HOA: v1
 States: 5