Doesn't seem to handle Bool negation well

Hi, just trying `@avx` for the first time on a simple loop that includes the negation of a boolean:
```
julia> xs = rand(10)

julia> function foo(x)
count = 0
@avx for i in eachindex(x)
count += !isnan(x[i])
end
count
end
foo (generic function with 1 method)

julia> foo(xs)
ERROR: MethodError: no method matching !(::UInt8)
Closest candidates are:
!(::Missing) at missing.jl:100
!(::Bool) at bool.jl:35
!(::Function) at operators.jl:880
...

```
Is that expected?

Reproduce the reported `foo` loop with `@avx`, `eachindex`, and `!isnan(x[i])`, then inspect the handling of boolean negation in the vectorization path. Done means determining whether this usage should work and documenting or correcting the behavior accordingly.

Vom Indexierungsmodell aus dem Issue-Text verfasst.