arch-power: Refactor rotate instructions

This renames the mask span fields and the rotate helper
of the base class.

Change-Id: I120006a0c052fcc34eb154a68d4b7f70a464df65
Signed-off-by: Sandipan Das <sandipan@linux.ibm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40929
Reviewed-by: Boris Shingarov <shingarov@labware.com>
Maintainer: Boris Shingarov <shingarov@labware.com>
Tested-by: kokoro <noreply+kokoro@google.com>
4 files changed