This operation also returns a tensor `idx` the same size as `x` that contains
the index of each value of `x` in the unique output `y`. In other words:

`y[idx[i]] = x[i] for i in [0, 1,...,rank(x) - 1]`

Examples:

```
# tensor 'x' is [1, 1, 2, 4, 4, 4, 7, 8, 8]
y, idx = unique(x)
y ==> [1, 2, 4, 7, 8]
idx ==> [0, 0, 1, 2, 2, 2, 3, 4, 4]
```

```

	v_1 := v.Args[1]
	v_0 := v.Args[0]
	b := v.Block
	// match: (IsInBounds idx len)
	// result: (SETB (CMPQ idx len))
	for {
		idx := v_0
		len := v_1
		v.reset(OpAMD64SETB)
		v0 := b.NewValue0(v.Pos, OpAMD64CMPQ, types.TypeFlags)
		v0.AddArg2(idx, len)
		v.AddArg(v0)
		return true
	}
}
func rewriteValueAMD64_OpIsNonNil(v *Value) bool {

	LGSQ (R11), DX                          // 490fb513
	LGSQ (BX), R11                          // 4c0fb51b
	LGSQ (R11), R11                         // 4d0fb51b
	LIDT (BX)                               // 0f011b
	LIDT (R11)                              // 410f011b
	LLDT (BX)                               // 0f0013
	LLDT (R11)                              // 410f0013

		v1.AuxInt = int32ToAuxInt(int32(t.Elem().Size()))
		v0.AddArg2(idx, v1)
		v.AddArg2(ptr, v0)
		return true
	}
	// match: (PtrIndex <t> ptr idx)
	// cond: config.PtrSize == 8
	// result: (AddPtr ptr (Mul64 <typ.Int> idx (Const64 <typ.Int> [t.Elem().Size()])))
	for {
		t := v.Type
		ptr := v_0
		idx := v_1
		if !(config.PtrSize == 8) {
			break
		}