[AArch64] Migrate from SearchableTable to GenericTable/Enum. NFC (llvm#121661)

SearchableTable is the legacy version that does not appear to be well
documented. Not sure if the plan was to delete it eventually.

We can eventually use the PrimaryKey feature of GenericTable to remove
one of the SearchIndex declarations. This will sort the generated table
by the primary key and remove the separately generated indexing table to
reduce .rodata size.

This patch is just the mechanical migration. The size savings will be
done in follow ups.

Author: topperc