Simplify computation of environment stack delta.

src/compiler/ast-graph-builder.h

Index: src/compiler/ast-graph-builder.h
diff --git a/src/compiler/ast-graph-builder.h b/src/compiler/ast-graph-builder.h
index c79d9c37a2ea0e25c3af5cfcef0f9d2d14355726..aa874be5bdff7122b26fb78568d49b8536d6eaf3 100644
--- a/src/compiler/ast-graph-builder.h
+++ b/src/compiler/ast-graph-builder.h
@@ -290,7 +290,7 @@ class AstGraphBuilder : public AstVisitor {
   void VisitForValues(ZoneList<Expression*>* exprs);
 
   // Common for all IterationStatement bodies.
-  void VisitIterationBody(IterationStatement* stmt, LoopBuilder* loop, int);
+  void VisitIterationBody(IterationStatement* stmt, LoopBuilder* loop);
 
   // Dispatched from VisitCallRuntime.
   void VisitCallJSRuntime(CallRuntime* expr);
@@ -400,6 +400,11 @@ class AstGraphBuilder::Environment : public ZoneObject {
     DCHECK(depth >= 0 && depth <= stack_height());
     values()->erase(values()->end() - depth, values()->end());
   }
+  void Trim(int trim_to_height) {
+    int depth = stack_height() - trim_to_height;
+    DCHECK(depth >= 0 && depth <= stack_height());
+    values()->erase(values()->end() - depth, values()->end());
+  }
 
   // Preserve a checkpoint of the environment for the IR graph. Any
   // further mutation of the environment will not affect checkpoints.