Direct access to intr->const_index[n], where different slots have
different meanings, is somewhat confusing.

Instead, let's put some extra info in nir_intrinsic_infos[] about which
slots map to what, and add some get/set helpers.  The helpers validate
that the field being accessed (base/writemask/etc) is applicable for the
intrinsic opc, for some extra safety.  And nir_print can use this to
dump out decoded const_index fields.

Signed-off-by: Rob Clark <robclark@freedesktop.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>