Clean up is-ASCII checks.

src/x64/regexp-macro-assembler-x64.cc

Index: src/x64/regexp-macro-assembler-x64.cc
diff --git a/src/x64/regexp-macro-assembler-x64.cc b/src/x64/regexp-macro-assembler-x64.cc
index 2cf85f11655c5da705bbf1f48956ebc30d7c6987..0838ddd4619629daf3f56a7614aa094995b2f1e0 100644
--- a/src/x64/regexp-macro-assembler-x64.cc
+++ b/src/x64/regexp-macro-assembler-x64.cc
@@ -223,9 +223,7 @@ void RegExpMacroAssemblerX64::CheckCharacters(Vector<const uc16> str,
   // If input is ASCII, don't even bother calling here if the string to
   // match contains a non-ascii character.
   if (mode_ == ASCII) {
-    for (int i = 0; i < str.length(); i++) {
-      ASSERT(str[i] <= String::kMaxAsciiCharCodeU);
-    }
+    ASSERT(String::IsAscii(str.start(), str.length()));
   }
 #endif
   int byte_length = str.length() * char_size();