On Arm64, the 31 encoding refers to either the zero register or the stack
pointer depending on the instruction. However, in order to distinguish them in
the assembler, the stack pointer has an internal code which isn't 31.

As a result, we need to use the internal code when converting a Register to a
DWARF code.

Bug: v8:6644
Change-Id: If8a6b6c94badd7d10c22664c7081733258b07a31
Reviewed-on: https://chromium-review.googlesource.com/866870Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Pierre Langlois <pierre.langlois@arm.com>
Cr-Commit-Position: refs/heads/master@{#50583}