HeapTupleHeaderAdvanceLatestRemovedXid doing the wrong thing with multixacts

I just noticed that HeapTupleHeaderAdvanceLatestRemovedXid is comparing Xmax as a TransactionId without verifying
whetherit is a multixact or not.  Since they advance separately, this could lead to bogus answers.  This probably needs
tobe fixed.  I didn't look into past releases to see if there's a live released bug here or not. 

I think the fix is simply to ignore the Xmax if the HEAP_XMAX_IS_MULTI bit is set.

Additionally I think it should check HEAP_XMAX_INVALID before reading the Xmax at all.

--
Álvaro Herrera (from some crappy webmail)