Make ToBooleanStub more consistent across platforms.

src/x64/full-codegen-x64.cc

Index: src/x64/full-codegen-x64.cc
===================================================================
--- src/x64/full-codegen-x64.cc	(revision 8353)
+++ src/x64/full-codegen-x64.cc	(working copy)
@@ -564,10 +564,10 @@
                                Label* if_true,
                                Label* if_false,
                                Label* fall_through) {
-  ToBooleanStub stub;
+  ToBooleanStub stub(result_register());
   __ push(result_register());
   __ CallStub(&stub);
-  __ testq(rax, rax);
+  __ testq(result_register(), result_register());
   // The stub returns nonzero for true.
   Split(not_zero, if_true, if_false, fall_through);
 }