cmd/compile: add rules to avoid unnecessary MOVDaddr for PPC64

cmd/compile: add rules to avoid unnecessary MOVDaddr for PPC64

This adds some rules to recognize MOVDaddr in those cases where
it is just adding 0 to a ptr value. Instead the ptr value can just
be used.

Change-Id: I95188defc9701165c86bbea70d14d037a9e54853
Reviewed-on: https://go-review.googlesource.com/c/go/+/527698
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Paul Murphy <murp@ibm.com>
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Than McIntosh <thanm@google.com>