Emit slightly more compact code in untag double.

src/assembler.cc

Index: src/assembler.cc
diff --git a/src/assembler.cc b/src/assembler.cc
index b0b44fd980e0721bc7e32f7f4ec6bacccf7dc911..de3711b341b33ca098a347188126237ee7b9e129 100644
--- a/src/assembler.cc
+++ b/src/assembler.cc
@@ -67,6 +67,7 @@ namespace internal {
 const double DoubleConstant::min_int = kMinInt;
 const double DoubleConstant::one_half = 0.5;
 const double DoubleConstant::minus_zero = -0.0;
+const double DoubleConstant::nan = OS::nan_value();
 const double DoubleConstant::negative_infinity = -V8_INFINITY;
 const char* RelocInfo::kFillerCommentString = "DEOPTIMIZATION PADDING";
 
@@ -745,6 +746,12 @@ ExternalReference ExternalReference::address_of_negative_infinity() {
 }
 
 
+ExternalReference ExternalReference::address_of_nan() {
+  return ExternalReference(reinterpret_cast<void*>(
+      const_cast<double*>(&DoubleConstant::nan)));
+}
+
+
 #ifndef V8_INTERPRETED_REGEXP
 
 ExternalReference ExternalReference::re_check_stack_guard_state() {