glsl: Refine the loop instruction counting.

glsl: Refine the loop instruction counting.

Before, we were only counting top-level instructions.  But if we have
an assignment of a giant expression tree (such as the ones eventually
generated by glsl-fs-unroll), we were counting the same as an
assignment of a variable deref.

glsl-fs-unroll-explosion now fails in a reasonable amount of time on
i965 because the unrolling didn't go ridiculously far.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>