[release-branch.go1.18] cmd/compile: fix boolean comparison on PPC64

[release-branch.go1.18] cmd/compile: fix boolean comparison on PPC64

Following CL 421457, for PPC64.

Should fix PPC64 builds.

Updates #52788.
Updates #53397.

Change-Id: I193ac31cfba18b4f907dd2523b51368251fd6fad
Reviewed-on: https://go-review.googlesource.com/c/go/+/405116
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/421459
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>