Simplify ArrayConcatNode

* The assert in the constructor proves it's always 2+ children.

Author: eregon

src/main/java/org/truffleruby/core/array/ArrayConcatNode.java

@@ -32,37 +32,14 @@ public ArrayConcatNode(RubyNode[] children) {
         this.children = children;
     }
 
+    @ExplodeLoop
     @Override
     public RubyArray execute(VirtualFrame frame) {
         if (arrayBuilderNode == null) {
             CompilerDirectives.transferToInterpreterAndInvalidate();
             arrayBuilderNode = insert(ArrayBuilderNode.create());
         }
-        if (children.length == 1) {
-            return executeSingle(frame);
-        } else {
-            return executeMultiple(frame);
-        }
-    }
-
-    private RubyArray executeSingle(VirtualFrame frame) {
-        BuilderState state = arrayBuilderNode.start();
-        final Object childObject = children[0].execute(frame);
 
-        final int size;
-        if (isArrayProfile.profile(childObject instanceof RubyArray)) {
-            final RubyArray childArray = (RubyArray) childObject;
-            size = childArray.size;
-            arrayBuilderNode.appendArray(state, 0, childArray);
-        } else {
-            size = 1;
-            arrayBuilderNode.appendValue(state, 0, childObject);
-        }
-        return createArray(arrayBuilderNode.finish(state, size), size);
-    }
-
-    @ExplodeLoop
-    private RubyArray executeMultiple(VirtualFrame frame) {
         BuilderState state = arrayBuilderNode.start();
         int length = 0;