nir: convert the glsl intrinsic image_size to nir_intrinsic_image_size

nir: convert the glsl intrinsic image_size to nir_intrinsic_image_size

v2, review from Francisco Jerez:
 - make the destination variable as large as what the nir instrinsic
   defines (4) instead of the size of the return variable of glsl. This
   is still safe for the already existing code because all the intrinsics
   affected returned the same amount of components as expected by glsl IR.
   In the case of image_size, it is not possible to do so because the
   returned number of component depends on the image type and this case
   is not well handled by nir.

v3:
- Style fix

Signed-off-by: Martin Peres <martin.peres@linux.intel.com>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>