Add test case for labeled break in const assignment

See #51350 for more information.

Author: rasendubi

src/test/run-pass/consts/const-labeled-break.rs

@@ -0,0 +1,10 @@
+// Using labeled break in a while loop has caused an illegal instruction being
+// generated, and an ICE later.
+//
+// See https://github.com/rust-lang/rust/issues/51350 for more information.
+
+const CRASH: () = 'a: while break 'a {};
+
+fn main() {
+    println!("{:?}", CRASH);
+}