X64: Many small tweaks and more usages of "load smi as int32".

src/x64/assembler-x64.h

 // Copyright (c) 1994-2006 Sun Microsystems Inc.
 // 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.
 //
@@ skipping 289 matching lines @@
           ScaleFactor scale,
           int32_t disp);
 
   // Offset from existing memory operand.
   // Offset is added to existing displacement as 32-bit signed values and
   // this must not overflow.
   Operand(const Operand& base, int32_t offset);
 
  private:
   byte rex_;
-  byte buf_[10];
+  byte buf_[6];
   // The number of bytes in buf_.
   unsigned int len_;
 
   // Set the ModR/M byte without an encoded 'reg' register. The
   // register is encoded later as part of the emit_operand operation.
   // set_modrm can be called before or after set_sib and set_disp*.
   inline void set_modrm(int mod, Register rm);
 
   // Set the SIB byte if one is needed. Sets the length to 2 rather than 1.
   inline void set_sib(ScaleFactor scale, Register index, Register base);
@@ skipping 1077 matching lines @@
  private:
   Assembler* assembler_;
 #ifdef DEBUG
   int space_before_;
 #endif
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_ASSEMBLER_X64_H_