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

src/arm/lithium-arm.cc

Index: src/arm/lithium-arm.cc
diff --git a/src/arm/lithium-arm.cc b/src/arm/lithium-arm.cc
index ea06064e5ebf2a0827594bc1389ee0bf7e7c9112..191183918cad1edffbe9c5094b5977a704d13083 100644
--- a/src/arm/lithium-arm.cc
+++ b/src/arm/lithium-arm.cc
@@ -2006,8 +2006,8 @@ 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)));
 }