cmd/internal/obj/arm64: allow transition from $0 to ZR for MSR
Previously the first operand of MSR could be $0, which would be
converted to the ZR register. This is prohibited by CL 404316,
this CL restores this instruction format.
Change-Id: I5b5be59e76aa58423a0fb96942d1b2a9de62e311
Reviewed-on: https://go-review.googlesource.com/c/go/+/426198
TryBot-Result: Gopher Robot <gobot@golang.org> Reviewed-by: David Chase <drchase@google.com> Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Eric Fang <eric.fang@arm.com>