RFR(S): 8189613: [MVT] __Value oop not preserved when safepointing at return

[Roland Westrelin]

> http://cr.openjdk.java.net/~thartmann/8189613/webrev.00/

Wouldn't it be cleaner to change is_returning_oop() so it returns true
if the method returns __Value or ValueTypeReturnedAsFields is disabled?

Roland.