Emit slightly more compact code in untag double.

src/assembler.h

Index: src/assembler.h
diff --git a/src/assembler.h b/src/assembler.h
index 8ebbfadf80f0c3bc88a991233434686986d086b4..b66421a2e72a1cb97137d45d66bbba441ac41fb0 100644
--- a/src/assembler.h
+++ b/src/assembler.h
@@ -53,6 +53,7 @@ class DoubleConstant: public AllStatic {
   static const double one_half;
   static const double minus_zero;
   static const double negative_infinity;
+  static const double nan;
 };
 
 
@@ -590,6 +591,7 @@ class ExternalReference BASE_EMBEDDED {
   static ExternalReference address_of_one_half();
   static ExternalReference address_of_minus_zero();
   static ExternalReference address_of_negative_infinity();
+  static ExternalReference address_of_nan();
 
   static ExternalReference math_sin_double_function();
   static ExternalReference math_cos_double_function();