all: convert PPC64 CMPx ...,R0,... to CMPx Rx,$0

all: convert PPC64 CMPx ...,R0,... to CMPx Rx,$0

Cleanup all remaining trivial compares against $0 in ppc64x assembly.

In math, SRD ...,Rx; CMP Rx, $0 is further simplified to SRDCC.

Change-Id: Ia2bc204953e32f08ee142bfd06a91965f30f99b6
Reviewed-on: https://go-review.googlesource.com/c/go/+/587016
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Lynn Boger <laboger@linux.vnet.ibm.com>
Run-TryBot: Paul Murphy <murp@ibm.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>