[IR] Remove an unnecessary cast (NFC) (#146381)

C is already of unsigned char.

Author: kazutakahirata

llvm/lib/IR/AsmWriter.cpp

@@ -447,8 +447,7 @@ void llvm::printLLVMNameWithoutPrefix(raw_ostream &OS, StringRef Name) {
       // in the range 0-255.  This is important when building with MSVC because
       // its implementation will assert.  This situation can arise when dealing
       // with UTF-8 multibyte characters.
-      if (!isalnum(static_cast<unsigned char>(C)) && C != '-' && C != '.' &&
-          C != '_') {
+      if (!isalnum(C) && C != '-' && C != '.' && C != '_') {
         NeedsQuotes = true;
         break;
       }