nir/opt_large_constants: Handle store writemasks

nir/opt_large_constants: Handle store writemasks

This fixes some piglit tests on radeonsi NIR where a varying is
initialized to a constant array in the vertex shader. Varying packing
after nir_lower_io_to_temporaries creates writemasked stores which
persist after pulling the constant initialization down into the fragment
shader.

While we're here, rewrite handle_constant_store() to do the loop over
components outside the switch, so that we don't have to duplicate the
writemask checking for every bitsize.

Fixes: 1235850522c ("nir: Add a large constants optimization pass")
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>