glsl: Skip loop-too-large heuristic if indexing arrays of a certain size

glsl: Skip loop-too-large heuristic if indexing arrays of a certain size

A pattern in certain shaders is:

   uniform vec4 colors[NUM_LIGHTS];

   for (int i = 0; i < NUM_LIGHTS; i++) {
      ...use colors[i]...
   }

In this case, the application author expects the shader compiler to
unroll the loop.  By doing so, it replaces variable indexing of the
array with constant indexing, which is more efficient.

This patch extends the heuristic to see if arrays accessed within the
loop are indexed by an induction variable, and if the array size exactly
matches the number of loop iterations.  If so, the application author
probably intended us to unroll it.  If not, we rely on the existing
loop-too-large heuristic.

Improves performance in a phong shading microbenchmark by 2.88x, and a
shadow mapping microbenchmark by 1.63x.  Without variable indexing, we
can upload the small uniform arrays as push constants instead of pull
constants, avoiding shader memory access.  Affects several games, but
doesn't appear to impact their performance.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Acked-by: Kristian Høgsberg <krh@bitplanet.net>