Add a PassScope for PrepareForRegisterAllocation

This pass does transform the graph so make it part of cfg-dumping.

Change-Id: I42e361382c85c97b974faad8bb0fcf2cb0750355
diff --git a/compiler/optimizing/optimizing_compiler.cc b/compiler/optimizing/optimizing_compiler.cc
index 13d6d62..c1b4d24 100644
--- a/compiler/optimizing/optimizing_compiler.cc
+++ b/compiler/optimizing/optimizing_compiler.cc
@@ -483,7 +483,11 @@
 static void AllocateRegisters(HGraph* graph,
                               CodeGenerator* codegen,
                               PassObserver* pass_observer) {
-  PrepareForRegisterAllocation(graph).Run();
+  {
+    PassScope scope(PrepareForRegisterAllocation::kPrepareForRegisterAllocationPassName,
+                    pass_observer);
+    PrepareForRegisterAllocation(graph).Run();
+  }
   SsaLivenessAnalysis liveness(graph, codegen);
   {
     PassScope scope(SsaLivenessAnalysis::kLivenessPassName, pass_observer);
diff --git a/compiler/optimizing/prepare_for_register_allocation.h b/compiler/optimizing/prepare_for_register_allocation.h
index c8b8b0d..c90724c 100644
--- a/compiler/optimizing/prepare_for_register_allocation.h
+++ b/compiler/optimizing/prepare_for_register_allocation.h
@@ -32,6 +32,9 @@
 
   void Run();
 
+  static constexpr const char* kPrepareForRegisterAllocationPassName =
+      "prepare_for_register_allocation";
+
  private:
   void VisitNullCheck(HNullCheck* check) OVERRIDE;
   void VisitDivZeroCheck(HDivZeroCheck* check) OVERRIDE;