cmd/compile: enable lateLower pass on PPC64

cmd/compile: enable lateLower pass on PPC64

This allows new rules to be added which would otherwise
greatly overcomplicate the generic rules, like CC opcode
conversion or zero register simplification.

Change-Id: I1533f0fa07815aff99ed8ab890077bd22a3bfbf5
Reviewed-on: https://go-review.googlesource.com/c/go/+/442595
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>