Two small fixes for the Math.sin and Math.cos optimizations.

src/ia32/codegen-ia32.cc

Index: src/ia32/codegen-ia32.cc
===================================================================
--- src/ia32/codegen-ia32.cc	(revision 2166)
+++ src/ia32/codegen-ia32.cc	(working copy)
@@ -4995,11 +4995,8 @@
 
   // Store the result in the allocated heap number.
   __ fstp_d(FieldOperand(heap_number.reg(), HeapNumber::kValueOffset));
-  // Pop the extra copy of the argument.
-  frame_->Pop();
-  // Push the result on the frame.
-  frame_->Push(&heap_number);
-  heap_number.Unuse();
+  // Replace the extra copy of the argument with the result.
+  frame_->SetElementAt(0, &heap_number);
   done.Jump();
 
   call_runtime.Bind();