RFR(XS) 8029441: assert(!((nmethod*)_cb)->is_deopt_pc(_pc)) failed: invariant broken

[dean.long at oracle.com]

https://bugs.openjdk.java.net/browse/JDK-8029441

http://cr.openjdk.java.net/~dlong/8029441/webrev/

This assert happens if we are unlucky enough to be at the start of the 
deopt handler when another thread is calling 
JavaThread::pd_get_top_frame_for_profiling() on the first thread. In 
this case, the frame isn't going to be patched, so the assert is not 
helping.  I removed the assert, but added a new assert to patch_pc() to 
make sure we don't try to patch an unpatchable frame.

Tested with "hs-nightly-runtime", "hs-tier0-comp", and all the 
individual tests where this crash was happening.

dl