RFR (XS): 8214523: Fix nmethod asserts for concurrent nmethod unloading

[Erik Österlund]

Hi,

There are some asserts in nmethod.cpp that do not play well with 
concurrent nmethod unloading. This patch adjusts said asserts and makes 
nmethod::maybe_invalidate_installed_code() return early if the nmethod 
is not JVMCI compiled, to dodge JVMCI requirements on locking, that is 
far from supported right now anyway for concurrent unloading.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8214523

Webrev:
http://cr.openjdk.java.net/~eosterlund/8214523/webrev.00/

Thanks,
/Erik