setBlockComptimeBody

    fn setBlockComptimeBody(gz: *GenZir, inst: Zir.Inst.Index, comptime_reason: std.zig.SimpleComptimeReason) !void {
        const astgen = gz.astgen;
        const gpa = astgen.gpa;
        const body = gz.instructionsSlice();
        const body_len = astgen.countBodyLenAfterFixups(body);

        const zir_tags = astgen.instructions.items(.tag);
        assert(zir_tags[@backingInt(inst)] == .block_comptime); // use `setBlockBody` instead

        try astgen.extra.ensureUnusedCapacity(
            gpa,
            @typeInfo(Zir.Inst.BlockComptime).@"struct".field_names.len + body_len,
        );
        const zir_datas = astgen.instructions.items(.data);
        zir_datas[@backingInt(inst)].pl_node.payload_index = astgen.addExtraAssumeCapacity(
            Zir.Inst.BlockComptime{
                .reason = comptime_reason,
                .body_len = body_len,
            },
        );
        astgen.appendBodyWithFixups(body);
        gz.unstack();
    }