// sbfx
// (x << lc) >> rc
(SRAconst [rc] (SLLconst [lc] x)) && lc <= rc => (SBFX [armBFAuxInt(rc-lc, 64-rc)] x)
// int64(x) >> rc
(SRAconst [rc] (MOVWreg x)) && rc < 32 => (SBFX [armBFAuxInt(rc, 32-rc)] x)
(SRAconst [rc] (MOVHreg x)) && rc < 16 => (SBFX [armBFAuxInt(rc, 16-rc)] x)
(SRAconst [rc] (MOVBreg x)) && rc < 8  => (SBFX [armBFAuxInt(rc,  8-rc)] x)

	v_0 := v.Args[0]
	// match: (Signmask x)
	// result: (SRAconst x [31])
	for {
		x := v_0
		v.reset(OpMIPSSRAconst)
		v.AuxInt = int32ToAuxInt(31)
		v.AddArg(x)
		return true
	}
}
func rewriteValueMIPS_OpSlicemask(v *Value) bool {
	v_0 := v.Args[0]
	b := v.Block
	// match: (Slicemask <t> x)
	// result: (SRAconst (NEG <t> x) [31])
	for {
		t := v.Type
		x := v_0