PSI Unifier: Unify references which correspond to the same declaration

#KT-5996 Fixed

idea/testData/refactoring/introduceVariable/callUnderSmartCast.kt

@@ -0,0 +1,11 @@
+trait T {
+    fun foo(): Int
+}
+
+trait U: T
+
+fun test(t: T): Int {
+    return if (t is U)
+        <selection>t.foo()</selection> + 1
+    else t.foo()
+}

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

@@ -0,0 +1,12 @@
+trait T {
+    fun foo(): Int
+}
+
+trait U: T
+
+fun test(t: T): Int {
+    val i = t.foo()
+    return if (t is U)
+        i + 1
+    else i
+}