[turbofan] reduce ResolveControlFlow overhead.

src/compiler/register-allocator.h

Index: src/compiler/register-allocator.h
diff --git a/src/compiler/register-allocator.h b/src/compiler/register-allocator.h
index 55b4cc59633dccf4c307e39db20465eb0013ea9f..dc3881de4777a3b46a2e513ddffb804cda8e9830 100644
--- a/src/compiler/register-allocator.h
+++ b/src/compiler/register-allocator.h
@@ -572,6 +572,8 @@ class TopLevelLiveRange final : public LiveRange {
                         : ++last_child_id_;
   }
 
+  int GetChildCount() const { return last_child_id_ + 1; }
+
   bool IsSpilledOnlyInDeferredBlocks() const {
     return spilled_in_deferred_blocks_;
   }