[TableGen] Extend direct lookup to instruction values in generic tables. (#80486)

Currently, for some tables involving a single primary key field which is
integral and densely numbered, a direct lookup is generated rather than
a binary search. This patch extends the direct lookup function
generation to instructions, where the integral value corresponds to the
instruction's enum value.

While this isn't as common as for other tables, it does occur in at
least one downstream backend and one in-tree backend.

Added a unit test and minimally updated the documentation.