RFR (XS): 8198560: Remove implicit casts from oop to JavaThread* and jlong*

[Erik Österlund]

Hi,

In the current state, oop may be implicitly casted to JavaThread* and 
jlong*. There does not appear to be code relying on these casts, and 
therefore they should be removed. In the unlikely case this operation 
would make sense, then you may use cast_from_oop<JavaThread*>(obj) instead.

Webrev:
http://cr.openjdk.java.net/~eosterlund/8198560/webrev.00/

Bug:
https://bugs.openjdk.java.net/browse/JDK-8198560

Thanks,
/Erik