Memoize types in is_representable to avoid exponential worst-case

I could have made representability a cached query, but that would have
been added complexity for not much benefit - outside of the exponential
worst-case, this pass is fast enough already.

Fixes #42747.

src/test/run-pass/issue-42747.rs

@@ -0,0 +1,55 @@
+// Copyright 2017 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.
+
+macro_rules! fooN {
+    ($cur:ident $prev:ty) => {
+        #[allow(dead_code)]
+        enum $cur {
+            Empty,
+            First($prev),
+            Second($prev),
+            Third($prev),
+            Fourth($prev),
+        }
+    }
+}
+
+fooN!(Foo0 ());
+fooN!(Foo1 Foo0);
+fooN!(Foo2 Foo1);
+fooN!(Foo3 Foo2);
+fooN!(Foo4 Foo3);
+fooN!(Foo5 Foo4);
+fooN!(Foo6 Foo5);
+fooN!(Foo7 Foo6);
+fooN!(Foo8 Foo7);
+fooN!(Foo9 Foo8);
+fooN!(Foo10 Foo9);
+fooN!(Foo11 Foo10);
+fooN!(Foo12 Foo11);
+fooN!(Foo13 Foo12);
+fooN!(Foo14 Foo13);
+fooN!(Foo15 Foo14);
+fooN!(Foo16 Foo15);
+fooN!(Foo17 Foo16);
+fooN!(Foo18 Foo17);
+fooN!(Foo19 Foo18);
+fooN!(Foo20 Foo19);
+fooN!(Foo21 Foo20);
+fooN!(Foo22 Foo21);
+fooN!(Foo23 Foo22);
+fooN!(Foo24 Foo23);
+fooN!(Foo25 Foo24);
+fooN!(Foo26 Foo25);
+fooN!(Foo27 Foo26);
+
+fn main() {
+    let _foo = Foo27::Empty;
+}