[crankshaft] only compile string index access with element key.

src/crankshaft/hydrogen.cc

Index: src/crankshaft/hydrogen.cc
diff --git a/src/crankshaft/hydrogen.cc b/src/crankshaft/hydrogen.cc
index 41b6c148c743c64bac72c1914585f08397e3063c..2f3c2a911536ea5f693b5f1419b81d69f33681cd 100644
--- a/src/crankshaft/hydrogen.cc
+++ b/src/crankshaft/hydrogen.cc
@@ -7876,7 +7876,7 @@ void HOptimizedGraphBuilder::PushLoad(Property* expr,
 void HOptimizedGraphBuilder::BuildLoad(Property* expr,
                                        BailoutId ast_id) {
   HInstruction* instr = NULL;
-  if (expr->IsStringAccess()) {
+  if (expr->IsStringAccess() && expr->GetKeyType() == ELEMENT) {
     HValue* index = Pop();
     HValue* string = Pop();
     HInstruction* char_code = BuildStringCharCodeAt(string, index);