zink: convert samplemask to array

The samplemask fragment-output is an uint[1] until nir_lower_io_arrays_to_elements_no_indirects removes the array. But for SPIR-V, we need it to be an array, so we've been manually rematerializing it while emitting code. This leads to a lot of small pain-points in the code-emitting.

So let's rematerialize the array as a NIR pass before emitting code to get rid of this complexity.