intel/fs: Add need_dest parameter to fs_visitor::nir_emit_alu

intel/fs: Add need_dest parameter to fs_visitor::nir_emit_alu

This is the same as the need_dest parameter to
prepare_alu_destination_and_sources.  This allows us to not change the
register that is expected to hold an result if an instruction is
re-emitted.  This is particularly a problem if the re-emitted
instruction is a partial write.  A later patch will use this feature.

No shader-db changes on any Intel platform.

v2: Don't do the Boolean resolve when there is no destination.  If the
ALU instruction didn't write a register, there's nothing to resolve.
This replaces an earlier patch "intel/fs: Allocate dummy destination
register when need_dest is false".

Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>