Add "unnecessary local variable" inspection #KT-15958 Fixed

idea/testData/inspectionsLocal/unnecessaryVariable/topLevelCopy.kt

@@ -0,0 +1,9 @@
+// PROBLEM: none
+
+val x: Number? = null
+
+fun foo(): Int {
+    val <caret>y = x
+    if (y is Int) return y
+    return 0
+}