Optimize loading the global object for a named load on x64.

This is a port of r3911 to x64.
Review URL: http://codereview.chromium.org/1983005

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@4642 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/x64/codegen-x64.cc

@@ -5251,6 +5251,11 @@ void CodeGenerator::LoadReference(Reference* ref) {
     // The expression is a variable proxy that does not rewrite to a
     // property.  Global variables are treated as named property references.
     if (var->is_global()) {
+      // If rax is free, the register allocator prefers it.  Thus the code
+      // generator will load the global object into rax, which is where
+      // LoadIC wants it.  Most uses of Reference call LoadIC directly
+      // after the reference is created.
+      frame_->Spill(rax);
       LoadGlobal();
       ref->set_type(Reference::NAMED);
     } else {