cmd/compile/internal/ppc64: map/slice literals janitoring

Simplify slice/map literal expressions.
Caught with gofmt -d -s, fixed with gofmt -w -s
Checked that the result can still be compiled with Go 1.4.

Change-Id: I201cd90fdfb8de2971c46ad7fce64111152b697e
Reviewed-on: https://go-review.googlesource.com/13832Reviewed-by: Andrew Gerrand <adg@golang.org>