[8u] RFR: Prune unneeded (jccb|jmpb)_if_possible

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/prune-if-possible/webrev.01/

This shows up as difference against 8u upstream. But this is not really needed, and we have actually
purged it in all other versions. See here:
  https://mail.openjdk.java.net/pipermail/shenandoah-dev/2018-June/006287.html

Testing: hotspot_gc_shenandoah

-- 
Thanks,
-Aleksey