(ANDconst [c] (MOVWZreg x)) => (ANDconst [c&0xFFFFFFFF] x)

// Eliminate unnecessary sign/zero extend following right shift
(MOV(B|H|W)Zreg (SRWconst [c] (MOVBZreg x))) => (SRWconst [c] (MOVBZreg x))
(MOV(H|W)Zreg (SRWconst [c] (MOVHZreg x))) => (SRWconst [c] (MOVHZreg x))
(MOVWZreg (SRWconst [c] (MOVWZreg x))) => (SRWconst [c] (MOVWZreg x))
(MOV(B|H|W)reg (SRAWconst [c] (MOVBreg x))) => (SRAWconst [c] (MOVBreg x))

// other bytes might contain junk so a zero extension is required if
// the desired output type is larger than 1 byte.
(SumBytes2 x) => (ADDW (SRWconst <typ.UInt8> x [8]) x)
(SumBytes4 x) => (SumBytes2 (ADDW <typ.UInt16> (SRWconst <typ.UInt16> x [16]) x))
(SumBytes8 x) => (SumBytes4 (ADDW <typ.UInt32> (SRDconst <typ.UInt32> x [32]) x))

(Bswap64 ...) => (MOVDBR ...)
(Bswap32 ...) => (MOVWBR ...)

// 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
	}

		{name: "SRDconst", argLength: 1, reg: gp11, asm: "SRD", aux: "UInt8"}, // unsigned arg0 >> auxint, shift amount 0-63
		{name: "SRWconst", argLength: 1, reg: gp11, asm: "SRW", aux: "UInt8"}, // unsigned uint32(arg0) >> auxint, shift amount 0-31

		// Arithmetic shifts clobber flags.