Lift assignment: do not report in cases where argument are of different types #KT-21520 Fixed

idea/testData/inspectionsLocal/liftOut/ifToAssignment/nullableMutableList.kt.after

@@ -0,0 +1,9 @@
+// WITH_RUNTIME
+fun test(b: Boolean, x: Int, y: Int?) {
+    val list = mutableListOf<Int?>()
+    list += if (b) {
+        x
+    } else {
+        y
+    }
+}