cmd/internal/obj/arm64: use more appropriate types for opldpstp
This allows for a large number of casts to be removed at call sites.
While here, use consistent register naming.
Change-Id: I68cee985f6500ed9523f7fb3efbc11bb849681dc
Reviewed-on: https://go-review.googlesource.com/c/go/+/471516 Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Joel Sing <joel@sing.id.au>
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: Michael Knyszek <mknyszek@google.com>