Optimization to reuse BindingContext in completion

idea/idea-completion/testData/incrementalResolve/noDataFlowFromOldStatement.kt

@@ -0,0 +1,10 @@
+fun foo(p: String?): String {
+    println()
+    bar(p!!, <before><change>)
+}
+
+fun bar(s: String, p: Int) { }
+
+// BACKSPACES: 4
+// TYPE: "."
+// EXIST: { itemText: "substring", attributes: "grayed" }