Fix shift-overflow in very old run-pass test.

Author: pnkfelix

src/test/run-pass/over-constrained-vregs.rs

@@ -11,7 +11,7 @@
 // Regression test for issue #152.
 pub fn main() {
     let mut b: uint = 1_usize;
-    while b <= 32_usize {
+    while b < std::mem::size_of::<usize>() {
         0_usize << b;
         b <<= 1_usize;
         println!("{}", b);