cmd/internal/obj/arm64: refactor the extended/shifted register encoding to the backend

cmd/internal/obj/arm64: refactor the extended/shifted register encoding to the backend

The current code encodes the register and the shift/extension into a.Offset
field and this is done in the frontend. The CL refactors it to have the
frontend record the register/shift/extension information in a.Reg or a.Index
and leave the encoding stuff for the backend.

Change-Id: I600f456aec95377b7b79cd58e94afcb30aca5d19
Reviewed-on: https://go-review.googlesource.com/106815
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>