Make BitField3 a raw uint32 field, and move to the start of the map.

src/arm64/macro-assembler-arm64.h

 // Copyright 2013 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_ARM64_MACRO_ASSEMBLER_ARM64_H_
 #define V8_ARM64_MACRO_ASSEMBLER_ARM64_H_
 
 #include <vector>
 
 #include "v8globals.h"
@@ skipping 813 matching lines @@
   inline void Move(Register dst, Register src) { Mov(dst, src); }
 
   void LoadInstanceDescriptors(Register map,
                                Register descriptors);
   void EnumLengthUntagged(Register dst, Register map);
   void EnumLengthSmi(Register dst, Register map);
   void NumberOfOwnDescriptors(Register dst, Register map);
 
   template<typename Field>
   void DecodeField(Register reg) {
-    static const uint64_t shift = Field::kShift + kSmiShift;
+    static const uint64_t shift = Field::kShift;
     static const uint64_t setbits = CountSetBits(Field::kMask, 32);
     Ubfx(reg, reg, shift, setbits);
   }
 
   // ---- SMI and Number Utilities ----
 
   inline void SmiTag(Register dst, Register src);
   inline void SmiTag(Register smi);
   inline void SmiUntag(Register dst, Register src);
   inline void SmiUntag(Register smi);
@@ skipping 1439 matching lines @@
 #error "Unsupported option"
 #define CODE_COVERAGE_STRINGIFY(x) #x
 #define CODE_COVERAGE_TOSTRING(x) CODE_COVERAGE_STRINGIFY(x)
 #define __FILE_LINE__ __FILE__ ":" CODE_COVERAGE_TOSTRING(__LINE__)
 #define ACCESS_MASM(masm) masm->stop(__FILE_LINE__); masm->
 #else
 #define ACCESS_MASM(masm) masm->
 #endif
 
 #endif  // V8_ARM64_MACRO_ASSEMBLER_ARM64_H_