Make register allocator to disregard constant computation with no uses.

runtime/vm/assembler_ia32_test.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"
 #if defined(TARGET_ARCH_IA32)
 
 #include "vm/assembler.h"
 #include "vm/os.h"
 #include "vm/unit_test.h"
@@ skipping 327 matching lines @@
 ASSEMBLER_TEST_GENERATE(Bitwise, assembler) {
   __ movl(ECX, Immediate(42));
   __ xorl(ECX, ECX);
   __ orl(ECX, Immediate(256));
   __ movl(EAX, Immediate(4));
   __ orl(ECX, EAX);
   __ movl(EAX, Immediate(0xfff0));
   __ andl(ECX, EAX);
   __ movl(EAX, Immediate(1));
   __ orl(ECX, EAX);
+  __ xorl(ECX, Immediate(0));
   __ movl(EAX, ECX);
   __ ret();
 }
 
 
 ASSEMBLER_TEST_RUN(Bitwise, entry) {
   typedef int (*Bitwise)();
   EXPECT_EQ(256 + 1, reinterpret_cast<Bitwise>(entry)());
 }
 
@@ skipping 1501 matching lines @@
                      FieldAddress(ECX, GrowableObjectArray::data_offset()),
                      EAX);
   __ popl(CTX);
   __ ret();
 }
 
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_IA32