Lift assignment out: do not suggest if other assignments available

So #KT-18709 Fixed

idea/testData/inspectionsLocal/liftOut/ifToAssignment/multipleAssignments.kt

@@ -0,0 +1,12 @@
+// PROBLEM: none
+
+fun foo(x: Boolean): String {
+    var s = ""
+    <caret>if (x) {
+        s += "a"
+        s += "b"
+    } else {
+        s += "c"
+    }
+    return s
+}