don't walk MIR if no local variables need liveness

This is true for tuple-stress and html5ever

Author: nikomatsakis

src/librustc_mir/borrow_check/nll/type_check/liveness/liveness_map.rs

@@ -86,6 +86,11 @@ impl NllLivenessMap {
             to_local,
         }
     }
+
+    /// True if there are no local variables that need liveness computation.
+    crate fn is_empty(&self) -> bool {
+        self.to_local.is_empty()
+    }
 }
 
 /// Index given to each local variable whose type contains a region.

src/librustc_mir/borrow_check/nll/type_check/liveness/mod.rs

@@ -55,7 +55,7 @@ pub(super) fn generate<'gcx, 'tcx>(
     let liveness = LivenessResults::compute(mir, &liveness_map);
 
     // For everything else, it is only live where it is actually used.
-    {
+    if !liveness_map.is_empty() {
         let mut generator = TypeLivenessGenerator {
             cx,
             mir,