(ANDconst [m] (SRWconst x [s])) && mergePPC64RShiftMask(m,s,32) == 0 => (MOVDconst [0])
(ANDconst [m] (SRWconst x [s])) && mergePPC64AndSrwi(m,s) != 0 => (RLWINM [mergePPC64AndSrwi(m,s)] x)
(AND (MOVDconst [m]) (SRWconst x [s])) && mergePPC64RShiftMask(m,s,32) == 0 => (MOVDconst [0])

// and return mask & m.
func mergePPC64RShiftMask(m, s, nbits int64) int64 {
	smask := uint64((1<<uint(nbits))-1) >> uint(s)
	return m & int64(smask)
}

// Combine (ANDconst [m] (SRWconst [s])) into (RLWINM [y]) or return 0
func mergePPC64AndSrwi(m, s int64) int64 {
	mask := mergePPC64RShiftMask(m, s, 32)
	if !isPPC64WordRotateMask(mask) {
		return 0
	}