RFR: More AArch64 assembler cleanups

[Roman Kennke]

This cleans up some more stuff in aarch64 assembler, and thus removes
diffs to upstream:
- One missing line
- A gratituous if () .. the exact same thing is handled in the switch a
few lines above. Infact, this is almost a bug: the code for the cmpoop
gets emitted 2x. Doesn't seem to hurt, but better to get rid of it.
- Some code in MacroAssembler::stop() that we changed back when we did
the store-checks stuff. We removed that stuff long ago, so let's also
remove this leftover.

With this patch, the aarch64-portion of Shenandoah is clean, except for
the push/pop FP register stuff, which we will upstream with Shenandoah.

http://cr.openjdk.java.net/~rkennke/more-aarch64-cleanups/webrev.00/

Testing: tier3_gc_shenandoah

Ok?

Roman