[AArch64] Use StringRef::contains_insensitive (NFC)

Author: kazutakahirata

llvm/lib/Target/AArch64/AsmParser/AArch64AsmParser.cpp

@@ -3858,7 +3858,7 @@ bool AArch64AsmParser::parseSysAlias(StringRef Name, SMLoc NameLoc,
 
   Lex(); // Eat operand.
 
-  bool ExpectRegister = (Op.lower().find("all") == StringRef::npos);
+  bool ExpectRegister = !Op.contains_insensitive("all");
   bool HasRegister = false;
 
   // Check for the optional register operand.