Only count nested returns when the outer return is reachable

This narrows the definition of nested returns such that only when the
outer return has a chance of being executed (due to the inner return
being conditional) do we mark the function as having nested returns.

Fixes #19684

src/librustc_driver/lib.rs

@@ -472,7 +472,7 @@ pub fn list_metadata(sess: &Session, path: &Path,
 /// The diagnostic emitter yielded to the procedure should be used for reporting
 /// errors of the compiler.
 pub fn monitor<F:FnOnce()+Send>(f: F) {
-    static STACK_SIZE: uint = 32000000; // 32MB
+    static STACK_SIZE: uint = 8 * 1024 * 1024; // 8MB
 
     let (tx, rx) = channel();
     let w = io::ChanWriter::new(tx);