Avoid erase_regions_ty queries if there are no regions to erase

It's overall faster to perform this extra check than to perform the
query, even if the result is already in the query cache.

src/librustc/ty/erase_regions.rs

@@ -21,6 +21,11 @@ impl<'a, 'gcx, 'tcx> TyCtxt<'a, 'gcx, 'tcx> {
     pub fn erase_regions<T>(self, value: &T) -> T
         where T : TypeFoldable<'tcx>
     {
+        // If there's nothing to erase avoid performing the query at all
+        if !value.has_erasable_regions() {
+            return value.clone();
+        }
+
         let value1 = value.fold_with(&mut RegionEraserVisitor { tcx: self });
         debug!("erase_regions({:?}) = {:?}", value, value1);
         value1