[LLVM][TableGen] Change X86MnemonicTables to use const RecordKeeper (#109053)
Change X86MnemonicTables to use const RecordKeeper.
This is a part of effort to have better const correctness in TableGen
backends:
https://discourse.llvm.org/t/psa-planned-changes-to-tablegen-getallderiveddefinitions-api-potential-downstream-breakages/81089