setCondBrPayload

fn setCondBrPayload(
    condbr: Zir.Inst.Index,
    cond: Zir.Inst.Ref,
    then_scope: *GenZir,
    else_scope: *GenZir,
) !void {
    defer then_scope.unstack();
    defer else_scope.unstack();
    const astgen = then_scope.astgen;
    const then_body = then_scope.instructionsSliceUpto(else_scope);
    const else_body = else_scope.instructionsSlice();
    const then_body_len = astgen.countBodyLenAfterFixups(then_body);
    const else_body_len = astgen.countBodyLenAfterFixups(else_body);
    try astgen.extra.ensureUnusedCapacity(
        astgen.gpa,
        @typeInfo(Zir.Inst.CondBr).@"struct".fields.len + then_body_len + else_body_len,
    );

    const zir_datas = astgen.instructions.items(.data);
    zir_datas[@intFromEnum(condbr)].pl_node.payload_index = astgen.addExtraAssumeCapacity(Zir.Inst.CondBr{
        .condition = cond,
        .then_body_len = then_body_len,
        .else_body_len = else_body_len,
    });
    astgen.appendBodyWithFixups(then_body);
    astgen.appendBodyWithFixups(else_body);
}