add test

Author: hargoniX

tests/lean/run/double_match.lean

@@ -0,0 +1,22 @@
+/--
+trace: [Compiler.saveMono] size: 6
+    def foo f x : Option Nat :=
+      let _x.1 := f x;
+      cases _x.1 : Option Nat
+      | Option.none =>
+        return _x.1
+      | Option.some val.2 =>
+        let _x.3 := Nat.add val.2 val.2;
+        let _x.4 := some ◾ _x.3;
+        return _x.4
+-/
+#guard_msgs in
+set_option trace.Compiler.saveMono true in
+def foo (f : Nat → Option Nat) (x : Nat) : Option Nat :=
+  if let some val := f x then
+    if let some val2 := f x then
+      some <| val + val2
+    else
+      none
+  else
+    none