glsl: Rework oversize array check for gl_TexCoord.

glsl: Rework oversize array check for gl_TexCoord.

The check now applies both when explicitly declaring the size of
gl_TexCoord and when implicitly setting the size of gl_TexCoord by
accessing it using integral constant expressions.

This is prep work for adding similar size checks to gl_ClipDistance.

Fixes piglit tests texcoord/implicit-access-max.{frag,vert}.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>