Prevent expression selector from selecting just a label

Previously, extract variable could act on e.g. 'break@outerLoop' and
extract the label, leading into nonsensical 'val x = @outerLoop'.
Prevent JetRefactoringUtil.selectExpression from returning label
expressions to fix this.

 #KT-4515 Fixed

idea/testData/expressionSelection/labelledStatement.kt

@@ -0,0 +1,7 @@
+fun f() {
+    @loop while (true) {
+        break@lo<selection>o</selection>p
+    }
+}
+// KT-4515: Extract variable can attempt to extract a label from a labelled statement.
+/* break@loop */

idea/testData/expressionSelection/labelledThis.kt

@@ -0,0 +1,6 @@
+class Foo() {
+    val s = this@F<selection>o</selection>o.toString()
+}
+
+// KT-4515: Extract variable can attempt to extract a label from a labelled statement.
+/* this@Foo */