Speed up adding literal chars when the buffer is known to be one-byte

src/parsing/scanner.cc

Index: src/parsing/scanner.cc
diff --git a/src/parsing/scanner.cc b/src/parsing/scanner.cc
index 6a9b32e63461f3b12e334546d8b94db6c48d49d7..f9bba099e296693d07928d4cb253043656ab4a37 100644
--- a/src/parsing/scanner.cc
+++ b/src/parsing/scanner.cc
@@ -858,7 +858,7 @@ Token::Value Scanner::ScanString() {
       Advance<false, false>();
       return Token::STRING;
     }
-    uc32 c = c0_;
+    char c = static_cast<char>(c0_);
     if (c == '\\') break;
     Advance<false, false>();
     AddLiteralChar(c);
@@ -1283,7 +1283,7 @@ Token::Value Scanner::ScanIdentifierOrKeyword() {
   LiteralScope literal(this);
   if (IsInRange(c0_, 'a', 'z')) {
     do {
-      uc32 first_char = c0_;
+      char first_char = static_cast<char>(c0_);
       Advance<false, false>();
       AddLiteralChar(first_char);
     } while (IsInRange(c0_, 'a', 'z'));
@@ -1291,11 +1291,11 @@ Token::Value Scanner::ScanIdentifierOrKeyword() {
     if (IsDecimalDigit(c0_) || IsInRange(c0_, 'A', 'Z') || c0_ == '_' ||
         c0_ == '$') {
       // Identifier starting with lowercase.
-      uc32 first_char = c0_;
+      char first_char = static_cast<char>(c0_);
       Advance<false, false>();
       AddLiteralChar(first_char);
       while (IsAsciiIdentifier(c0_)) {
-        uc32 first_char = c0_;
+        char first_char = static_cast<char>(c0_);
         Advance<false, false>();
         AddLiteralChar(first_char);
       }
@@ -1313,7 +1313,7 @@ Token::Value Scanner::ScanIdentifierOrKeyword() {
     HandleLeadSurrogate();
   } else if (IsInRange(c0_, 'A', 'Z') || c0_ == '_' || c0_ == '$') {
     do {
-      uc32 first_char = c0_;
+      char first_char = static_cast<char>(c0_);
       Advance<false, false>();
       AddLiteralChar(first_char);
     } while (IsAsciiIdentifier(c0_));