Offset bitvectors fail to vectorise

LoopVectorization.check_args returns false for `OffsetArray{Bool, BitVector}`, but doesn't with the similar `OffsetArray{Bool, Vector{Bool}}`
Code to reproduce the problem:
```julia
using OffsetArrays
using LoopVectorization

OffsetArray(zeros(Bool, 10)) |> LoopVectorization.check_args
OffsetArray(trues(10)) |> LoopVectorization.check_args
```
Both are expected to return `true`

Start by running the OffsetArrays and LoopVectorization reproduction in the issue and compare check_args for OffsetArray(zeros(Bool, 10)) with OffsetArray(trues(10)). Trace the check_args path for BitVector versus Vector{Bool}; done means both examples return true.

Scritto dal modello di indicizzazione a partire dal testo della issue.