RFR: Remove C2 write-barrier from .ad files

[Roman Kennke]

I propose to get rid of write-barriers in C2 .ad files, and instead
always expand WBs in ideal graph. We've been doing this for quite a
while now by default, with seemingly no ill effects.

In next step, this will make it easier to move
MacroAssembler::shenandoah_write_barrier() away into gc/shenandoah
subfolder.

http://cr.openjdk.java.net/~rkennke/remove-c2-wb/webrev.01/

Testing: tier1_gc_shenandoah

Ok?

Roman