Replace ToAsciiVector and ToUC16Vector with single function that returns a tagged value.

src/objects-inl.h

Index: src/objects-inl.h
diff --git a/src/objects-inl.h b/src/objects-inl.h
index c5fda89e42efdf368c5300de5ef34ef7b19e4009..098bd7a5c7b7f81db8d7814d9e9a3b6df820ad45 100644
--- a/src/objects-inl.h
+++ b/src/objects-inl.h
@@ -297,6 +297,11 @@ StringRepresentationTag StringShape::representation_tag() {
 }
 
 
+uint32_t StringShape::encoding_tag() {
+  return type_ & kStringEncodingMask;
+}
+
+
 uint32_t StringShape::full_representation_tag() {
   return (type_ & (kStringRepresentationMask | kStringEncodingMask));
 }