Auto merge of #32210 - Aatch:mir-traversal, r=nikomatsakis

rBreak Critical Edges and other MIR work

This PR is built on top of #32080.

This adds the basic depth-first traversals for MIR, preorder, postorder and reverse postorder. The MIR blocks are now translated using reverse postorder. There is also a transform for breaking critical edges, which includes the edges from `invoke`d calls (`Drop` and `Call`), to account for the fact that we can't add code after an `invoke`. It also stops generating the intermediate block (since the transform essentially does it if necessary already).

The kinds of cases this deals with are difficult to produce, so the test is the one I managed to get. However, it seems to bootstrap with `-Z orbit`, which it didn't before my changes.

src/test/run-pass/mir_trans_critical_edge.rs

@@ -0,0 +1,53 @@
+// 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.
+
+// This code produces a CFG with critical edges that, if we don't
+// handle properly, will cause invalid codegen.
+
+#![feature(rustc_attrs)]
+
+enum State {
+    Both,
+    Front,
+    Back
+}
+
+pub struct Foo<A: Iterator, B: Iterator> {
+    state: State,
+    a: A,
+    b: B
+}
+
+impl<A, B> Foo<A, B>
+where A: Iterator, B: Iterator<Item=A::Item>
+{
+    // This is the function we care about
+    #[rustc_mir]
+    fn next(&mut self) -> Option<A::Item> {
+        match self.state {
+            State::Both => match self.a.next() {
+                elt @ Some(..) => elt,
+                None => {
+                    self.state = State::Back;
+                    self.b.next()
+                }
+            },
+            State::Front => self.a.next(),
+            State::Back => self.b.next(),
+        }
+    }
+}
+
+// Make sure we actually translate a version of the function
+pub fn do_stuff(mut f: Foo<Box<Iterator<Item=u32>>, Box<Iterator<Item=u32>>>) {
+    let _x = f.next();
+}
+
+fn main() {}