Introduce Variable: Do not insert unused variable references

#KT-7227 Fixed

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

@@ -1,6 +1,5 @@
 fun a() {
     val i = 1
     i
-    i
     if (true) i
 }

idea/testData/refactoring/introduceVariable/UsedAndUnusedExpressions1.kt

@@ -0,0 +1,10 @@
+class A {
+    fun test() = 1
+}
+
+fun main(args: Array<String>) {
+    val a = A()
+    <selection>a.test()</selection>
+
+    val b = a.test()
+}

idea/testData/refactoring/introduceVariable/UsedAndUnusedExpressions1.kt.after

@@ -0,0 +1,10 @@
+class A {
+    fun test() = 1
+}
+
+fun main(args: Array<String>) {
+    val a = A()
+    val i = a.test()
+
+    val b = i
+}

idea/testData/refactoring/introduceVariable/UsedAndUnusedExpressions2.kt

@@ -0,0 +1,10 @@
+class A {
+    fun test() = 1
+}
+
+fun main(args: Array<String>) {
+    val a = A()
+    a.test()
+
+    val b = <selection>a.test()</selection>
+}

idea/testData/refactoring/introduceVariable/UsedAndUnusedExpressions2.kt.after

@@ -0,0 +1,10 @@
+class A {
+    fun test() = 1
+}
+
+fun main(args: Array<String>) {
+    val a = A()
+    val i = a.test()
+
+    val b = i
+}