Clean up is-ASCII checks.

src/string-search.h

Index: src/string-search.h
diff --git a/src/string-search.h b/src/string-search.h
index eac84757ecf1d45458133442c5cfc200627cd691..5de3c0951e5d37a1f06d399c4d7919efa5d9019c 100644
--- a/src/string-search.h
+++ b/src/string-search.h
@@ -66,12 +66,7 @@ class StringSearchBase {
   }
 
   static inline bool IsAsciiString(Vector<const uc16> string) {
-    for (int i = 0, n = string.length(); i < n; i++) {
-      if (static_cast<unsigned>(string[i]) > String::kMaxAsciiCharCodeU) {
-        return false;
-      }
-    }
-    return true;
+    return String::IsAscii(string.start(), string.length());
   }
 
   // The following tables are shared by all searches.