Optimize plug_leaks.

This commit avoids the `fold_regions` call in `plug_leaks` when
`skol_map` is empty, which is the common case. This gives speed-ups of
up to 1.14x on some of the rustc-benchmarks.

Author: nnethercote

src/librustc/infer/higher_ranked/mod.rs

@@ -756,6 +756,10 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
                skol_map,
                value);
 
+        if skol_map.is_empty() {
+            return self.resolve_type_vars_if_possible(value);
+        }
+
         // Compute a mapping from the "taint set" of each skolemized
         // region back to the `ty::BoundRegion` that it originally
         // represented. Because `leak_check` passed, we know that
@@ -813,9 +817,6 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             }
         });
 
-        debug!("plug_leaks: result={:?}",
-               result);
-
         self.pop_skolemized(skol_map, snapshot);
 
         debug!("plug_leaks: result={:?}", result);