Errors with certain simple for loop

Depending on how the index ranges are specified, there are a few cases in which I get unexpected errors.

For example, I am getting errors on something like this:

```julia
using LoopVectorization
a = zeros(3,2)
N = 2
@turbo thread=true for i in 1:1, j in 1:N
a[i,j] += 1.0
end
```

```julia
ERROR: MethodError: no method matching vmul_nw(::Int64)
```

The following also errors:

```julia
v = zeros(3)
N1 = 1
N2 = 3
@turbo thread=true for i in 1:N2, j in 1:N1
v[i,j] += 1.0
end
```

```julia
ERROR: BoundsError: attempt to access 1-element Vector{Any} at index [2]
```