[LLVM][TableGen] Change DAGISelEmitter to use const RecordKeeper (#109175)
Change DAGISelEmitter 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