Allow the optimizing code generator to call Math.pow with untagged doubles.

src/assembler.h

Index: src/assembler.h
===================================================================
--- src/assembler.h	(revision 5940)
+++ src/assembler.h	(working copy)
@@ -50,6 +50,7 @@
  public:
   static const double min_int;
   static const double one_half;
+  static const double negative_infinity;
 };
 
 
@@ -549,6 +550,7 @@
   // Static variables containing common double constants.
   static ExternalReference address_of_min_int();
   static ExternalReference address_of_one_half();
+  static ExternalReference address_of_negative_infinity();
 
   Address address() const {return reinterpret_cast<Address>(address_);}