RFR (S): 8153439: do not install an empty SpeculationLog in an nmethod

[Doug Simon]

The JVMCI speculation log mechanism might not be used by a specific JVMCI compiler or JVMCI compiler configuration. In this case, the nmethod::_speculation_log field should be initialized to NULL to reduce unnecessary processing of the field by the GC.

https://bugs.openjdk.java.net/browse/JDK-8153439
http://cr.openjdk.java.net/~dnsimon/8153439