Simplify StringCharCodeAt in non-crankshaft codegen.

src/ia32/lithium-codegen-ia32.cc

Index: src/ia32/lithium-codegen-ia32.cc
diff --git a/src/ia32/lithium-codegen-ia32.cc b/src/ia32/lithium-codegen-ia32.cc
index f8459fc0f610a50e58a331bb8512e0a6e617bd23..8eec9c4329822d22baa347ece8640ec7ac4f09e9 100644
--- a/src/ia32/lithium-codegen-ia32.cc
+++ b/src/ia32/lithium-codegen-ia32.cc
@@ -3451,6 +3451,9 @@ void LCodeGen::DoStringCharCodeAt(LStringCharCodeAt* instr) {
 
   // Check whether the string is sequential. The only non-sequential
   // shapes we support have just been unwrapped above.
+  // Note that if the original string is a cons or slice with an external
+  // string as underlying string, we pass that unpacked underlying string with
+  // the updated index to the runtime function.
   __ bind(&check_sequential);
   STATIC_ASSERT(kSeqStringTag == 0);
   __ test(result, Immediate(kStringRepresentationMask));