Tentative implementation of string slices (hidden under the flag --string-slices).

src/x64/lithium-x64.cc

Index: src/x64/lithium-x64.cc
diff --git a/src/x64/lithium-x64.cc b/src/x64/lithium-x64.cc
index 5b502a80c91b1ca40226c17d08e2d0786897fe1f..9628e3cebca1e563e91f31738396605b003308b5 100644
--- a/src/x64/lithium-x64.cc
+++ b/src/x64/lithium-x64.cc
@@ -1989,8 +1989,8 @@ LInstruction* LChunkBuilder::DoStringAdd(HStringAdd* instr) {
 
 
 LInstruction* LChunkBuilder::DoStringCharCodeAt(HStringCharCodeAt* instr) {
-  LOperand* string = UseRegister(instr->string());
-  LOperand* index = UseRegisterOrConstant(instr->index());
+  LOperand* string = UseTempRegister(instr->string());
+  LOperand* index = UseTempRegister(instr->index());
   LStringCharCodeAt* result = new LStringCharCodeAt(string, index);
   return AssignEnvironment(AssignPointerMap(DefineAsRegister(result)));
 }