PSI Pattern Matching: Fix matching of !!

#KT-6021 Fixed

idea/testData/refactoring/introduceVariable/notNullAssertion.kt

@@ -0,0 +1,8 @@
+fun foo(): Int {
+    val s: String? = ""
+    return if (true) {
+        <selection>s!!.length</selection>
+    } else {
+        s!!.length
+    }
+}

idea/testData/refactoring/introduceVariable/notNullAssertion.kt.after

@@ -0,0 +1,9 @@
+fun foo(): Int {
+    val s: String? = ""
+    val i = s!!.length
+    return if (true) {
+        i
+    } else {
+        i
+    }
+}