[utils/TableGen/X86CompressEVEXTablesEmitter.cpp] Make sure the tablegen...

[utils/TableGen/X86CompressEVEXTablesEmitter.cpp] Make sure the tablegen output for the `checkPredicate` function is deterministic (#84533)

The output for the `checkPredicate` function was depending on a
`std::map` iteration that was non-deterministic from run to run, because
the keys were pointer values.
    
Make a change so that the keys are `StringRef`s so the ordering is
stable.