nir/gather_info: Move setting uses_64bit out of the switch

nir/gather_info: Move setting uses_64bit out of the switch

Otherwise, as we add things to the switch, we're going to forget and add
some 64-bit op at some point in the future and it'll stop getting
flagged.  There's no reason why we can't do the check for derivatives.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>