Improve HtmlTokenizer.AtToken performance. (dotnet/razor#1880)

Our razor typing test measured 153 CPU ms in this method. Optimized by fewer calls to CurrentCharacter, not checking '<' twice, and uswing a switch stmt.\n\nCommit migrated from dotnet/razor@c601c2f

Author: ToddGrun

src/Razor/Microsoft.AspNetCore.Razor.Language/src/Legacy/HtmlTokenizer.cs

@@ -253,21 +253,27 @@ private SyntaxToken Newline()
             return EndToken(SyntaxKind.NewLine);
         }
 
-        private bool AtToken()
+        private bool AtToken()
         {
-            return CurrentCharacter == '<' ||
-                   CurrentCharacter == '<' ||
-                   CurrentCharacter == '!' ||
-                   CurrentCharacter == '/' ||
-                   CurrentCharacter == '?' ||
-                   CurrentCharacter == '[' ||
-                   CurrentCharacter == '>' ||
-                   CurrentCharacter == ']' ||
-                   CurrentCharacter == '=' ||
-                   CurrentCharacter == '"' ||
-                   CurrentCharacter == '\'' ||
-                   CurrentCharacter == '@' ||
-                   (CurrentCharacter == '-' && Peek() == '-');
+            switch (CurrentCharacter)
+            {
+                case '<':
+                case '!':
+                case '/':
+                case '?':
+                case '[':
+                case '>':
+                case ']':
+                case '=':
+                case '"':
+                case '\'':
+                case '@':
+                    return true;
+                case '-':
+                    return Peek() == '-';
+            }
+
+            return false;
         }
 
         private StateResult Transition(HtmlTokenizerState state)