RFR: 8202640: Small C1 cleanups for BarrierSetC1

[Erik Österlund]

Hi,

After rebasing ZGC to BarrierSetC1, we noticed a few small cleanups that 
would be great.

* restore_live_registers_except_rax should be exposed so that barrier 
set backends may use it
* save_live_registers_no_oop_map has an unused argument that should be 
removed
* LIR_OpVisitState::visit should allow visiting LIR with a virtual call 
(for GC barrier LIR_Ops).
* LIR_List::append ought to be public so that barrier set backends can 
add barrier LIR_Ops.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8202640

Webrev:
http://cr.openjdk.java.net/~eosterlund/8202640/webrev.00/

Thanks,
/Erik