// li -1, rx; rlic rx,rx,sh,mb.
// Masks can also extend from the msb and wrap to
// the lsb too. That is, the valid masks are 32 bit strings
// of the form: 0..01..10..0 or 1..10..01..1 or 1...1
func isPPC64DoublewordRotateMask(v64 int64) bool {
	// Isolate rightmost 1 (if none 0) and add.
	v := uint64(v64)
	vp := (v & -v) + v
	// Likewise, for the wrapping case.
	vn := ^v
	vpn := (vn & -vn) + vn