util/set: Add a _mesa_set_search_or_add() function

util/set: Add a _mesa_set_search_or_add() function

Unlike _mesa_set_search_and_add(), it doesn't replace an entry if it's
found, returning it instead. This is useful for nir_instr_set, where
we have to know both the original original instruction and its
equivalent.

Reviewed-by: Eric Anholt <eric@anholt.net>
Acked-by: Jason Ekstrand <jason@jlekstrand.net>