[8u20] RFR 8038212: Method::is_valid_method() check has performance regression impact for stackwalking

[Coleen Phillimore]

Summary: Only prune metaspace virtual spaces at safepoint so walking 
them is safe outside a safepoint.

This change has been in hs-rt for a week and backported cleanly. JFR 
performance in 8u20 needs this fix.

open webrev at http://cr.openjdk.java.net/~coleenp/8038212_8u20/
bug link https://bugs.openjdk.java.net/browse/JDK-8038212

Thanks,
Coleen