cmd/compile: clean up ADDconst on PPC64

cmd/compile: clean up ADDconst on PPC64

ADDconst op is no longer used for addresses, as we lower Addr to
MOVDaddr. There is no rule that produces ADDconst with a non-nil
sym. So we can remove the sym aux field in ADDconst and limit its
use for adding constant (not offset to symbol).

Passes "toolstash -cmp" on std cmd for GOARCH=ppc64 and ppc64le.

Change-Id: Icee35cdb34d8d121ad7035076dfd07595c7ff809
Reviewed-on: https://go-review.googlesource.com/69450
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>