[scanner] Faster SkipMultiLineComment by avoiding a copy.

Avoid coping the last character to a local variable, by checking the parsing
state in a different order.

BUG=v8:7926

Change-Id: I0b62f711674beac8c81a25dd566a5ed0d681948b
Reviewed-on: https://chromium-review.googlesource.com/1148456
Commit-Queue: Florian Sattler <sattlerf@google.com>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54657}

src/parsing/scanner.cc

@@ -555,28 +555,30 @@ void Scanner::TryToParseSourceURLComment() {
   }
 }
 
-
 Token::Value Scanner::SkipMultiLineComment() {
   DCHECK_EQ(c0_, '*');
   Advance();
 
   while (c0_ != kEndOfInput) {
-    uc32 ch = c0_;
-    Advance();
     DCHECK(!unibrow::IsLineTerminator(kEndOfInput));
-    if (unibrow::IsLineTerminator(ch)) {
+    if (!has_multiline_comment_before_next_ && unibrow::IsLineTerminator(c0_)) {
       // Following ECMA-262, section 7.4, a comment containing
       // a newline will make the comment count as a line-terminator.
       has_multiline_comment_before_next_ = true;
     }
+
+    while (V8_UNLIKELY(c0_ == '*')) {
+      Advance();
       // If we have reached the end of the multi-line comment, we
       // consume the '/' and insert a whitespace. This way all
       // multi-line comments are treated as whitespace.
-    if (ch == '*' && c0_ == '/') {
+      if (c0_ == '/') {
         c0_ = ' ';
         return Token::WHITESPACE;
       }
     }
+    Advance();
+  }
 
   // Unterminated multi-line comment.
   return Token::ILLEGAL;