X64: Remove restriction on using r12 as index register.

src/x64/assembler-x64.cc

 // 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 78 matching lines @@
     set_modrm(2, base);
     set_disp32(disp);
   }
 }
 
 
 Operand::Operand(Register base,
                  Register index,
                  ScaleFactor scale,
                  int32_t disp): rex_(0) {
-  ASSERT(!index.is(rsp) && !index.is(r12));
+  ASSERT(!index.is(rsp));
   len_ = 1;
   set_sib(scale, index, base);
   if (disp == 0 && !base.is(rbp) && !base.is(r13)) {
-    // The call to set_modrm doesn't overwrite the REX.B bit possibly set
-    // by set_sib.
+    // This call to set_modrm doesn't overwrite the REX.B (or REX.X) bits
+    // possibly set by set_sib.
     set_modrm(0, rsp);
   } else if (is_int8(disp)) {
     set_modrm(1, rsp);
     set_disp8(disp);
   } else {
     set_modrm(2, rsp);
     set_disp32(disp);
   }
 }
 
@@ skipping 1473 matching lines @@
   UNIMPLEMENTED();
   return NULL;
 }
 
 Object* StubCompiler::CompileLazyCompile(Code::Flags a) {
   UNIMPLEMENTED();
   return NULL;
 }
 
 } }  // namespace v8::internal