MIPS: Simplify StringCharCodeAt in non-crankshaft codegen.

src/mips/lithium-codegen-mips.cc

Index: src/mips/lithium-codegen-mips.cc
diff --git a/src/mips/lithium-codegen-mips.cc b/src/mips/lithium-codegen-mips.cc
index fee9dadc2135b450f6697b8fc1de462dd3ab7bc1..5d9e7fccbf03365ab5b53b6ddd4b17b0bf1c6fad 100644
--- a/src/mips/lithium-codegen-mips.cc
+++ b/src/mips/lithium-codegen-mips.cc
@@ -3569,6 +3569,9 @@ 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);
   __ And(temp, result, Operand(kStringRepresentationMask));