vmap gives incorrect results for iszero

If I run vmap with certain functions i get wrong results in some cases. I noticed this first for an integer array with iszero:
```
a = rand(Int8,200,20)
@test map(iszero,a) == vmap(iszero,a) #fails

```
Repeated evaluations of `vmap(iszero,a)` give different answers each time.
The same happens for some other functions I tested such as:

```
a = rand(200,20) #also for floats
vmap( ==(2.0), a)
vmap( >(0.5), a)
```
As a pattern these all return boolean arrays, functions that return floats seem to be unaffected.

vmapreduce is unaffected:
```
a = rand( Int8,200,20)
@test vmapreduce(iszero,+,a) == mapreduce(iszero,+,a) #passes
@test sum(map(iszero,a)) == sum(vmap(iszero,a)) #fails
```
The inplace version seems affected as well:
```
a = rand( Int8,200,20)
out = rand( Bool,200,20)
vmap!(iszero,out,a)
@test out == map(iszero,a) #fails
```
Interestingly this seems to consistently return the same (wrong) result instead of the always changing values of the out of place version. Here again this seems to only affect functions that return bools.

This is just from some playing around not something I actually need but unless I'm violating some assumptions LoopVectorization makes this seems like a bug.

提供されている `vmap(iszero, a)` と `vmap!` の例で不一致を再現し、それぞれの結果を `map` と比較してください。さらに、`vmapreduce` が引き続き正しいことも確認してください。`vmap` と `vmap!` のエントリポイントから開始し、ブール値を返す関数を追跡してから、報告されたケースで `map` と等しい安定した結果を示す回帰テストを追加してください。

索引モデルが issue の本文から書いたものです。