freedreno/ir3: limit # of tex prefetch by shader size

freedreno/ir3: limit # of tex prefetch by shader size

It seems for short frag shaders, too much prefetch can be detrimental.
I think what we *really* want to do is decide after pre-RA sched, when
we also know about nop's and what the actual ir3 instruction count is.
But that will require re-working how prefetch lowering works.  For now
this is a super crude heuristic to attempt to approximate a good
solution.

Signed-off-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4923>