RFR 8u: Cherry-pick 8140584: nmethod::oops_do_marking_epilogue always runs verification code

[Aleksey Shipilev]

Hi,

shenandoah-8u runs into the serious performance problem with code cache scan --
we are wasting several milliseconds per pause -- and it seems to be fixed in 8u152:
  https://bugs.openjdk.java.net/browse/JDK-8140584

I propose we cherry-pick it to our shenandoah-8u repo, while we wait for it to
arrive from upstream.

Andrew, any objections?

Thanks,
-Aleksey