8065985: Inlining failure of Number.doubleValue() in JSType.toNumeric() causes 15% peak perf regresion on Box2D

Reviewed-by: lagergren, hannesw

nashorn/src/jdk.scripting.nashorn/share/classes/jdk/nashorn/internal/runtime/JSType.java

@@ -714,6 +714,9 @@ public enum JSType {
      * @return a number
      */
     public static double toNumber(final Object obj) {
+        if (obj instanceof Double) {
+            return (Double)obj;
+        }
         if (obj instanceof Number) {
             return ((Number)obj).doubleValue();
         }