glsl: When assigning to a whole array, mark the array as accessed.

glsl: When assigning to a whole array, mark the array as accessed.

The vs-varying-array-mat2-col-row-wr test writes a mat2[3] constant to
a mat2[3] varying out array, and also statically accesses element 1 of
it on the VS and FS sides.  At link time it would get trimmed down to
just 2 elements, and then codegen of the VS would end up generating
assignments to the unallocated last entry of the array.  On the new
i965 VS backend, that happened to land on the vertex position.

Some issues remain in this test on softpipe, i965/old-vs and
i965/new-vs on visual inspection, but i965 is passing because only one
green pixel is probed, not the whole split green/red quad.