Fix a braino in the stack layout.

Also do some refactoring to have this code be just in CodeGenerator.

Change-Id: I88de109889138af8d60027973c12a64bee813cb7
diff --git a/compiler/optimizing/nodes.h b/compiler/optimizing/nodes.h
index 689aab0..ca846b3 100644
--- a/compiler/optimizing/nodes.h
+++ b/compiler/optimizing/nodes.h
@@ -138,6 +138,10 @@
     return number_of_in_vregs_;
   }
 
+  uint16_t GetNumberOfLocalVRegs() const {
+    return number_of_vregs_ - number_of_in_vregs_;
+  }
+
   const GrowableArray<HBasicBlock*>& GetReversePostOrder() const {
     return reverse_post_order_;
   }