Auto merge of #24330 - pnkfelix:issue-24267, r=nikomatsakis

Extend rustc::middle::dataflow to allow filtering kills from flow-exits.

Fix borrowck analysis so that it will not treat a break that pops through an assignment
```rust
x = { ... break; ... }
```
as a kill of the "moved-out" bit for `x`.

Fix #24267.

[breaking-change], but really, its only breaking code that was already buggy.

src/test/compile-fail/issue-24267-flow-exit.rs

@@ -0,0 +1,29 @@
+// Copyright 2015 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.
+
+// Ensure that we reject code when a nonlocal exit (`break`,
+// `continue`) causes us to pop over a needed assignment.
+
+pub fn main() {
+    foo1();
+    foo2();
+}
+
+pub fn foo1() {
+    let x: i32;
+    loop { x = break; }
+    println!("{}", x); //~ ERROR use of possibly uninitialized variable: `x`
+}
+
+pub fn foo2() {
+    let x: i32;
+    for _ in 0..10 { x = continue; }
+    println!("{}", x); //~ ERROR use of possibly uninitialized variable: `x`
+}