setBoolBrBody

    fn setBoolBrBody(gz: *GenZir, bool_br: Zir.Inst.Index, bool_br_lhs: Zir.Inst.Ref) !void {
        const astgen = gz.astgen;
        const gpa = astgen.gpa;
        const body = gz.instructionsSlice();
        const body_len = astgen.countBodyLenAfterFixups(body);
        try astgen.extra.ensureUnusedCapacity(
            gpa,
            @typeInfo(Zir.Inst.BoolBr).@"struct".field_names.len + body_len,
        );
        const zir_datas = astgen.instructions.items(.data);
        zir_datas[@backingInt(bool_br)].pl_node.payload_index = astgen.addExtraAssumeCapacity(Zir.Inst.BoolBr{
            .lhs = bool_br_lhs,
            .body_len = body_len,
        });
        astgen.appendBodyWithFixups(body);
        gz.unstack();
    }