Fix compilation error in ia32 platform valgrind tests. Error was introduced in r8721.

src/ia32/cpu-ia32.cc

Index: src/ia32/cpu-ia32.cc
===================================================================
--- src/ia32/cpu-ia32.cc	(revision 8728)
+++ src/ia32/cpu-ia32.cc	(working copy)
@@ -1,4 +1,4 @@
-// Copyright 2006-2008 the V8 project authors. All rights reserved.
+// Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
@@ -67,7 +67,8 @@
   // solution is to run valgrind with --smc-check=all, but this comes at a big
   // performance cost.  We can notify valgrind to invalidate its cache.
 #ifdef VALGRIND_DISCARD_TRANSLATIONS
-  VALGRIND_DISCARD_TRANSLATIONS(start, size);
+  unsigned res = VALGRIND_DISCARD_TRANSLATIONS(start, size);
+  USE(res);
 #endif
 }