[arm64] Fix some signed/unsigned type inconsistencies.

test/cctest/test-assembler-arm64.cc

 // Copyright 2013 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:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 8129 matching lines @@
   SETUP();
   RegisterDump before;
 
   START();
   // Preserve the system stack pointer, in case we clobber it.
   __ Mov(x30, csp);
   // Initialize the other registers used in this test.
   uint64_t literal_base = 0x0100001000100101UL;
   __ Mov(x0, 0);
   __ Mov(x1, literal_base);
-  for (unsigned i = 2; i < x30.code(); i++) {
+  for (int i = 2; i < x30.code(); i++) {
     __ Add(Register::XRegFromCode(i), Register::XRegFromCode(i-1), x1);
   }
   before.Dump(&masm);
 
   // All of these instructions should be NOPs in these forms, but have
   // alternate forms which can write into the stack pointer.
   __ add(xzr, x0, x1);
   __ add(xzr, x1, xzr);
   __ add(xzr, xzr, x1);
 
@@ skipping 3171 matching lines @@
   __ Mov(x0, 1);
 
   END();
 
   RUN();
 
   CHECK_EQUAL_64(0x1, x0);
 
   TEARDOWN();
 }