i965: Assert array index on access to vec4_visitor's arrays.

i965: Assert array index on access to vec4_visitor's arrays.

v2: vec4_visitor::pack_uniform_registers(): Use correct comparison in the
  assert, this->uniforms is already adjusted. Compare the actual value used to
  index uniform_size and uniform_vector_size instead.

Signed-off-by: Petri Latvala <petri.latvala@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>