Refactor Hydrogen GVN into an HPhase and use the phase zone.

src/compiler.h

Index: src/compiler.h
diff --git a/src/compiler.h b/src/compiler.h
index 390a1d8d1abe47e79d040643bc2389af5b218470..161f40458c99638d74b9d5e7ae3470f7c87e1a94 100644
--- a/src/compiler.h
+++ b/src/compiler.h
@@ -626,7 +626,8 @@ class CompilationPhase BASE_EMBEDDED {
   bool ShouldProduceTraceOutput() const;
 
   const char* name() const { return name_; }
-  Isolate* isolate() const { return info_->isolate(); }
+  CompilationInfo* info() const { return info_; }
+  Isolate* isolate() const { return info()->isolate(); }
   Zone* zone() { return &zone_; }
 
  private: