Auto merge of #38833 - arielb1:constant-mir-overflow, r=eddyb

fix promotion of MIR terminators

promotion of MIR terminators used to try to promote the destination it
is trying to promote, leading to stack overflow.

Also clean up the code in `promote_temp` a bit to make it more understandable.

Fixes #37991.

cc @nikomatsakis
r? @eddyb

src/test/run-pass/issue-37991.rs

@@ -0,0 +1,20 @@
+// Copyright 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.
+
+#![feature(const_fn)]
+
+const fn foo() -> i64 {
+    3
+}
+
+fn main() {
+    let val = &(foo() % 2);
+    assert_eq!(*val, 1);
+}