glsl/dead_builin_varyings: Fix gl_FragData array lowering

glsl/dead_builin_varyings: Fix gl_FragData array lowering

The current implementation looks for array dereferences on gl_FragData and
immediately proceeds to lower them, however this is not enough because we
can have array access on vector variables too, like in this code:

out vec4 color;
void main()
{
   int i;
   for (i = 0; i < 4; i++)
      color[i] = 1.0;
}

Fix it by making sure that the actual variable being dereferenced is an array.

Fixes a crash in:
spec/arb_gpu_shader_fp64/execution/built-in-functions/fs-ldexp-dvec4.shader_test

Reviewed-by: Tapani Pälli <tapani.palli@intel.com>