introduce liveness constraints into NLL code

And do a bunch of gratuitious refactoring that I did not bother to
separate into nice commits.

src/test/mir-opt/nll/region-liveness-basic.rs

@@ -0,0 +1,55 @@
+// Copyright 2012-2016 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.
+
+// Basic test for liveness constraints: the region (`R1`) that appears
+// in the type of `p` includes the points after `&v[0]` up to (but not
+// including) the call to `use_x`. The `else` branch is not included.
+
+// compile-flags:-Znll -Zverbose
+//                     ^^^^^^^^^ force compiler to dump more region information
+
+#![allow(warnings)]
+
+fn use_x(_: usize) -> bool { true }
+
+fn main() {
+    let mut v = [1, 2, 3];
+    let p = &v[0];
+    if true {
+        use_x(*p);
+    } else {
+        use_x(22);
+    }
+}
+
+// END RUST SOURCE
+// START rustc.node12.nll.0.mir
+// | R1: {bb1[1], bb2[0], bb2[1]}
+// ...
+//             let _2: &'_#1r usize;
+// END rustc.node12.nll.0.mir
+// START rustc.node12.nll.0.mir
+//    bb1: {
+//        | Live variables here: [_1, _3]
+//        _2 = &'_#0r _1[_3];
+//        | Live variables here: [_2]
+//        switchInt(const true) -> [0u8: bb3, otherwise: bb2];
+//    }
+// END rustc.node12.nll.0.mir
+// START rustc.node12.nll.0.mir
+//    bb2: {
+//        | Live variables here: [_2]
+//        StorageLive(_7);
+//        | Live variables here: [_2]
+//        _7 = (*_2);
+//        | Live variables here: [_7]
+//        _6 = const use_x(_7) -> bb4;
+//    }
+// END rustc.node12.nll.0.mir