Make killing of out-of-scope borrows a pre-statement effect

Fixes #46875.
Fixes #46917.
Fixes #46935.

src/test/ui/nll/borrow-use-issue-46875.rs

@@ -0,0 +1,30 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(nll)]
+
+// run-pass
+
+fn vec() {
+    let mut _x = vec!['c'];
+    let _y = &_x;
+    _x = Vec::new();
+}
+
+fn int() {
+    let mut _x = 5;
+    let _y = &_x;
+    _x = 7;
+}
+
+fn main() {
+    vec();
+    int();
+}