nir/builder: Emit better code for iadd/imul_imm

nir/builder: Emit better code for iadd/imul_imm

Because we already know the immediate right-hand parameter, we can
potentially save the optimizer a bit of work.

Reviewed-by: Karol Herbst <kherbst@redhat.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>