[turbofan] Allocate a small amount of slack space to graph builder environments.

R=mstarzinger@chromium.org
BUG=

Review URL: https://codereview.chromium.org/709463003

Cr-Commit-Position: refs/heads/master@{#25200}
git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25200 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/compiler/graph-builder.cc

@@ -125,7 +125,11 @@ StructuredGraphBuilder::Environment::Environment(const Environment& copy)
     : builder_(copy.builder()),
       control_dependency_(copy.control_dependency_),
       effect_dependency_(copy.effect_dependency_),
-      values_(copy.values_) {}
+      values_(copy.zone()) {
+  const size_t kStackEstimate = 7;  // optimum from experimentation!
+  values_.reserve(copy.values_.size() + kStackEstimate);
+  values_.insert(values_.begin(), copy.values_.begin(), copy.values_.end());
+}
 
 
 void StructuredGraphBuilder::Environment::Merge(Environment* other) {