X64: Remove restriction on using r12 as index register.

src/x64/assembler-x64-inl.h

 // Copyright 2009 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 242 matching lines @@
   ASSERT((mod & -4) == 0);
   buf_[0] = (mod << 6) | (rm.code() & 0x7);
   // Set REX.B to the high bit of rm.code().
   rex_ |= (rm.code() >> 3);
 }
 
 
 void Operand::set_sib(ScaleFactor scale, Register index, Register base) {
   ASSERT(len_ == 1);
   ASSERT(is_uint2(scale));
-  // Use SIB with no index register only for base rsp or r12.
+  // Use SIB with no index register only for base rsp or r12. Otherwise we
+  // would skip the SIB byte entirely.
   ASSERT(!index.is(rsp) || base.is(rsp) || base.is(r12));
   buf_[1] = scale << 6 | (index.code() & 0x7) << 3 | (base.code() & 0x7);
   rex_ |= (index.code() >> 3) << 1 | base.code() >> 3;
   len_ = 2;
 }
 
 void Operand::set_disp8(int disp) {
   ASSERT(is_int8(disp));
   ASSERT(len_ == 1 || len_ == 2);
   int8_t* p = reinterpret_cast<int8_t*>(&buf_[len_]);
   *p = disp;
   len_ += sizeof(int8_t);
 }
 
 void Operand::set_disp32(int disp) {
   ASSERT(len_ == 1 || len_ == 2);
   int32_t* p = reinterpret_cast<int32_t*>(&buf_[len_]);
   *p = disp;
   len_ += sizeof(int32_t);
 }
 
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_ASSEMBLER_X64_INL_H_