cmd/internal/obj/arm64: replace range checks with isaddcon2

cmd/internal/obj/arm64: replace range checks with isaddcon2

isaddcon2 tests for the range 0 <= v <= 0xffffff - replace duplicated range
checks with calls to isaddcon2.

Change-Id: Ia6f331852ed3d77715b265cb4fcc500579eac711
Reviewed-on: https://go-review.googlesource.com/c/go/+/650935
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Fannie Zhang <Fannie.Zhang@arm.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>