nir: Add nir_builder support for individual system value loads.

nir: Add nir_builder support for individual system value loads.

The previous nir_load_system_value(b, nir_intrinsic_load_whatever), 0) was
rather verbose, when system values should be easy to generate.

The index is left out because only one system value had an index included
in it.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>