cmd/compile: remove branch in atomicCas{32,64} sequence on ppc64x.

cmd/compile: remove branch in atomicCas{32,64} sequence on ppc64x.

This removes one of the branches in the atomicCas sequences for
ppc64/ppc64le.

Change-Id: Ibb46fbfdce052889c69e3da298f28caff540d99b
Reviewed-on: https://go-review.googlesource.com/c/go/+/422014
Reviewed-by: Archana Ravindar <aravind5@in.ibm.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Lynn Boger <laboger@linux.vnet.ibm.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Paul Murphy <murp@ibm.com>