nir/lower_non_uniform: Better handle non-derefs

nir/lower_non_uniform: Better handle non-derefs

In particular, if we have an index or bindless handle we were passing
the original handle which, technically, is uniform within the context of
the if.  However, we can save the back-end compiler some effort if we
pass it the result of the read_first_invocation().

(Rebased by Kenneth Graunke and Rhys Perry.)

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7592>