When disassembling some instructions we would print `r12`, which can be
confusing when the rest of the disassembly consistently uses `ip`.

Bug: 
Change-Id: Id4cfc5805ef102a0845cdaaa8390e618ee981b19
Reviewed-on: https://chromium-review.googlesource.com/570038Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Cr-Commit-Position: refs/heads/master@{#46628}