Fix node class names for NilClass#nil? and Kernel#nil?

Author: eregon

src/main/java/org/truffleruby/core/bool/NilClassNodes.java

@@ -21,7 +21,7 @@ public abstract class NilClassNodes {
 
     /** Needs to be in Java for {@link InlinedIsNilNode} */
     @CoreMethod(names = "nil?", needsSelf = false)
-    public abstract static class AndNode extends CoreMethodNode {
+    public abstract static class IsNilNode extends CoreMethodNode {
 
         @Specialization
         protected boolean isNil() {

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

@@ -1270,7 +1270,7 @@ protected DynamicObject methodsSingleton(VirtualFrame frame, Object self, boolea
     }
 
     @CoreMethod(names = "nil?", needsSelf = false)
-    public abstract static class NilNode extends CoreMethodArrayArgumentsNode {
+    public abstract static class IsNilNode extends CoreMethodArrayArgumentsNode {
 
         @Specialization
         protected boolean isNil() {