TableGen/SearchableTables: Support more generic enums and tables
This is essentially a rewrite of the backend which introduces TableGen
base classes GenericEnum, GenericTable, and SearchIndex. They allow
generating custom enums and tables with lookup functions using
separately defined records as the underlying database.
Also added as part of this change:
- Lookup functions may use indices composed of multiple fields.
- Instruction fields are supported similar to Intrinsic fields.
- When the lookup key has contiguous numeric values, the lookup function will directly index into the table instead of using a binary search.
The existing SearchableTable functionality is internally mapped to the
Reviewers: arsenm, tra, t.p.northover
Subscribers: wdng, llvm-commits
Differential Revision: https://reviews.llvm.org/D48013