cmd/compile/internal/ssa: on PPC64, generate large constant paddi

This is only supported power10/linux/PPC64. This generates smaller,
faster code by merging a pli + add into paddi.

Change-Id: I1f4d522fce53aea4c072713cc119a9e0d7065acc
Reviewed-on: https://go-review.googlesource.com/c/go/+/531717


Run-TryBot: Paul Murphy <murp@ibm.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>