[arm64] Fix some signed/unsigned type inconsistencies.

src/arm64/assembler-arm64-inl.h

Index: src/arm64/assembler-arm64-inl.h
diff --git a/src/arm64/assembler-arm64-inl.h b/src/arm64/assembler-arm64-inl.h
index 15085d5c643e87ada0cfd5e14d142cc884d0a4a2..6de7fb1b2ac4c22e8a169f71ca676ccec836aa84 100644
--- a/src/arm64/assembler-arm64-inl.h
+++ b/src/arm64/assembler-arm64-inl.h
@@ -41,7 +41,7 @@ void RelocInfo::set_target_address(Address target,
 }
 
 
-inline unsigned CPURegister::code() const {
+inline int CPURegister::code() const {
   DCHECK(IsValid());
   return reg_code;
 }
@@ -54,12 +54,12 @@ inline CPURegister::RegisterType CPURegister::type() const {
 
 
 inline RegList CPURegister::Bit() const {
-  DCHECK(reg_code < (sizeof(RegList) * kBitsPerByte));
+  DCHECK(static_cast<size_t>(reg_code) < (sizeof(RegList) * kBitsPerByte));
   return IsValid() ? 1UL << reg_code : 0;
 }
 
 
-inline unsigned CPURegister::SizeInBits() const {
+inline int CPURegister::SizeInBits() const {
   DCHECK(IsValid());
   return reg_size;
 }