gallivm: Report the unsupported intrinsic instead of just assert(0);

Tripped over this in rendermode with softpipe NIR.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6567>

src/gallium/auxiliary/gallivm/lp_bld_nir.c

@@ -1581,6 +1581,9 @@ static void visit_intrinsic(struct lp_build_nir_context *bld_base,
       visit_interp(bld_base, instr, result);
       break;
    default:
+      fprintf(stderr, "Unsupported intrinsic: ");
+      nir_print_instr(&instr->instr, stderr);
+      fprintf(stderr, "\n");
       assert(0);
       break;
    }