nir: Add pass to optimize intrinsics

Specifically, constant fold intrinsics from ARB_shader_group_vote, but I
suspect it'll be useful for other things in the future.

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

src/intel/compiler/brw_nir.c

@@ -553,6 +553,7 @@ nir_optimize(nir_shader *nir, const struct brw_compiler *compiler,
       OPT(nir_opt_dce);
       OPT(nir_opt_cse);
       OPT(nir_opt_peephole_select, 0);
+      OPT(nir_opt_intrinsics);
       OPT(nir_opt_algebraic);
       OPT(nir_opt_constant_folding);
       OPT(nir_opt_dead_cf);