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

src/math.js

Index: src/math.js
===================================================================
--- src/math.js	(revision 5948)
+++ src/math.js	(working copy)
@@ -264,6 +264,7 @@
   %SetMathFunctionId($Math.round, 2);
   %SetMathFunctionId($Math.abs, 4);
   %SetMathFunctionId($Math.sqrt, 0xd);
+  %SetMathFunctionId($Math.pow, 0xe);
   // TODO(erikcorry): Set the id of the other functions so they can be
   // optimized.
 };