aarch64: warn on unpredictable results for new rcpc3 instructions

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.