Add regression test for #20626

Closes #20626.

Author: jooert

src/test/run-make/issue-20626/Makefile

@@ -0,0 +1,8 @@
+-include ../tools.mk
+
+# Test output to be four
+# The original error only occurred when printing, not when comparing using assert!
+
+all:
+	$(RUSTC) foo.rs -O
+	[ `$(call RUN,foo)` = "4" ]

src/test/run-make/issue-20626/foo.rs

@@ -0,0 +1,23 @@
+// 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.
+
+fn identity(a: &u32) -> &u32 { a }
+
+fn print_foo(f: &fn(&u32) -> &u32, x: &u32) {
+    print!("{}", (*f)(x));
+}
+
+fn main() {
+    let x = &4;
+    let f: fn(&u32) -> &u32 = identity;
+
+    // Didn't print 4 on optimized builds
+    print_foo(&f, x);
+}