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

[Roland Westrelin]

> http://cr.openjdk.java.net/~thartmann/8189613/webrev.01/

Looks reasonable. Maybe is_returning_oop() should be renamed to
maybe_returns_oop() (or something similar) given its answer is inexact.

Roland.