Clean up is-ASCII checks.

src/ia32/regexp-macro-assembler-ia32.cc

Index: src/ia32/regexp-macro-assembler-ia32.cc
diff --git a/src/ia32/regexp-macro-assembler-ia32.cc b/src/ia32/regexp-macro-assembler-ia32.cc
index d435a707758f06afe9ed87c0a14399221250b639..cc2f30dc1285df19d25d4ef0d746b149c08969c0 100644
--- a/src/ia32/regexp-macro-assembler-ia32.cc
+++ b/src/ia32/regexp-macro-assembler-ia32.cc
@@ -211,9 +211,7 @@ void RegExpMacroAssemblerIA32::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();