Make register allocator to disregard constant computation with no uses.

runtime/vm/assembler_x64_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_X64)
 
 #include "vm/assembler.h"
 #include "vm/os.h"
 #include "vm/unit_test.h"
@@ skipping 532 matching lines @@
   __ j(NOT_EQUAL, &error);
   __ xorq(RCX, RCX);
   __ orq(RCX, Immediate(256));
   __ movq(RAX, Immediate(4));
   __ orq(RCX, RAX);
   __ movq(RAX, Immediate(0xfff0));
   __ andq(RCX, RAX);
   __ movq(RAX, Immediate(1));
   __ pushq(RAX);
   __ orq(RCX, Address(RSP, 0));
+  __ xorq(RCX, Immediate(0));
   __ popq(RAX);
   __ movq(RAX, RCX);
   __ ret();
   __ Bind(&error);
   __ movq(RAX, Immediate(-1));
   __ ret();
 }
 
 
 ASSEMBLER_TEST_RUN(Bitwise64, entry) {
@@ skipping 1143 matching lines @@
   EXPECT_EQ(0, res);
   res = reinterpret_cast<ExtractSignBits>(entry)(-1.0);
   EXPECT_EQ(1, res);
   res = reinterpret_cast<ExtractSignBits>(entry)(-0.0);
   EXPECT_EQ(1, res);
 }
 
 }  // namespace dart
 
 #endif  // defined TARGET_ARCH_X64