Stop mw/core hhvm job in check, do them postmerge

The hhvm job is slowing down reporting and does not provide any added
value for now.  Instead trigger it on postmerge for REL1_23 and master
branches (though non voting).

Bug: 64687
Change-Id: I96d7a20c1b9dd2e5fe7dbd270ce60e8c31263318