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 <f682d884382604a3c853f1eb5e063792ab430218@in.ibm.com>
Reviewed-by: David Chase <ae171625adc8645b8b79b7e405a6da5aba5341e0@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Lynn Boger <b27445fe6fb259c37b8e0a4ce56e22e2413754bf@linux.vnet.ibm.com>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Reviewed-by: Paul Murphy <2e1d8d6f81223fe22f04c5349bc2b02555a275af@ibm.com>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go