Fix specializations in Kernel#hash to cover all Ruby values

(cherry picked from commit ef7c3dc)

Author: eregon

src/main/java/org/truffleruby/core/kernel/KernelNodes.java

@@ -933,7 +933,6 @@ protected boolean isFrozen(Object self,
     @CoreMethod(names = "hash")
     public abstract static class HashNode extends CoreMethodArrayArgumentsNode {
 
-
         public static HashNode create() {
             return KernelNodesFactory.HashNodeFactory.create(null);
         }
@@ -965,17 +964,12 @@ protected long hashBignum(RubyBignum value) {
             return HashOperations.hashBignum(value, getContext(), this);
         }
 
-        @Specialization
-        protected int hash(Nil self) {
+        @Specialization(guards = "!isRubyBignum(self)")
+        protected int hash(ImmutableRubyObject self) {
             return System.identityHashCode(self);
         }
 
         @Specialization
-        protected int hashEncoding(RubyEncoding self) {
-            return System.identityHashCode(self);
-        }
-
-        @Specialization(guards = "!isRubyBignum(self)")
         protected int hash(RubyDynamicObject self) {
             return System.identityHashCode(self);
         }