The previous patch for the feature rcpc3 introduced 4 new operations 
(ldiapp, stilp, ldapr, stlr).
The specification mentions some cases of inputs causing unpredictable 
results. gas currently fails
to diagnose them, and does not emit warnings. Even if the instruction 
encoding is valid, the
developer probably wants to know for those cases that the instruction 
won't have the expected
effect.
- ldiapp & stilp:
   * unpredictable load pair transfer with register overlap
   * unpredictable transfer with writeback
- ldapr & stlr:
   * unpredictable transfer with writeback

This patch also completes the existing relevant tests.

Regression tested on aarch64-none-elf, and no regression found.

Ok for binutils-master? I don't have commit access so I need someone
to commit on my behalf.

Regards,
Matthieu.