Handle loop carried dependencies that are apparent from constant offsets

```julia
using LoopVectorization
function grad!(𝛥rec, 𝛥ℛ, rec)
for i in 2:size(𝛥rec, 1)-1
for j in 2:size(𝛥rec, 2)-1
ℰ𝓍1 = conj(𝛥ℛ[1])
ℰ𝓍2 = rec[i, j + 1] + rec[i, j - 1]
ℰ𝓍3 = rec[i + 1, j] + ℰ𝓍2
ℰ𝓍4 = rec[i - 1, j] + ℰ𝓍3
ℰ𝓍5 = 4 * rec[i, j]
ℰ𝓍6 = ℰ𝓍4 - ℰ𝓍5
ℰ𝓍7 = 2ℰ𝓍6
ℰ𝓍8 = ℰ𝓍1 * ℰ𝓍7
ℰ𝓍9 = conj(ℰ𝓍8)
ℰ𝓍10 = rec[i - 1, j] + ℰ𝓍3
ℰ𝓍11 = ℰ𝓍10 - ℰ𝓍5
ℰ𝓍12 = 2ℰ𝓍11
ℰ𝓍13 = ℰ𝓍1 * ℰ𝓍12
ℰ𝓍14 = conj(ℰ𝓍13)
ℰ𝓍15 = rec[i - 1, j] + ℰ𝓍3
ℰ𝓍16 = ℰ𝓍15 - ℰ𝓍5
ℰ𝓍17 = 2ℰ𝓍16
ℰ𝓍18 = ℰ𝓍1 * ℰ𝓍17
ℰ𝓍19 = conj(ℰ𝓍18)
ℰ𝓍20 = rec[i - 1, j] + ℰ𝓍3
ℰ𝓍21 = ℰ𝓍20 - ℰ𝓍5
ℰ𝓍22 = 2ℰ𝓍21
ℰ𝓍23 = ℰ𝓍1 * ℰ𝓍22
ℰ𝓍24 = conj(ℰ𝓍23)
ℰ𝓍25 = rec[i - 1, j] + ℰ𝓍3
ℰ𝓍26 = ℰ𝓍25 - ℰ𝓍5
ℰ𝓍27 = 2ℰ𝓍26
ℰ𝓍28 = -4ℰ𝓍27
ℰ𝓍29 = ℰ𝓍1 * ℰ𝓍28
ℰ𝓍30 = conj(ℰ𝓍29)
𝛥rec[i - 1, j] = 𝛥rec[i - 1, j] + ℰ𝓍9
𝛥rec[i + 1, j] = 𝛥rec[i + 1, j] + ℰ𝓍14
𝛥rec[i, j + 1] = 𝛥rec[i, j + 1] + ℰ𝓍19
𝛥rec[i, j - 1] = 𝛥rec[i, j - 1] + ℰ𝓍24
𝛥rec[i, j] = 𝛥rec[i, j] + ℰ𝓍30
end
end
end
function gradavx!(𝛥rec, 𝛥ℛ, rec)
@avx for i in 2:size(𝛥rec, 1)-1
for j in 2:size(𝛥rec, 2)-1
ℰ𝓍1 = conj(𝛥ℛ[1])
ℰ𝓍2 = rec[i, j + 1] + rec[i, j - 1]
ℰ𝓍3 = rec[i + 1, j] + ℰ𝓍2
ℰ𝓍4 = rec[i - 1, j] + ℰ𝓍3
ℰ𝓍5 = 4 * rec[i, j]
ℰ𝓍6 = ℰ𝓍4 - ℰ𝓍5
ℰ𝓍7 = 2ℰ𝓍6
ℰ𝓍8 = ℰ𝓍1 * ℰ𝓍7
ℰ𝓍9 = conj(ℰ𝓍8)
ℰ𝓍10 = rec[i - 1, j] + ℰ𝓍3
ℰ𝓍11 = ℰ𝓍10 - ℰ𝓍5
ℰ𝓍12 = 2ℰ𝓍11
ℰ𝓍13 = ℰ𝓍1 * ℰ𝓍12
ℰ𝓍14 = conj(ℰ𝓍13)
ℰ𝓍15 = rec[i - 1, j] + ℰ𝓍3
ℰ𝓍16 = ℰ𝓍15 - ℰ𝓍5
ℰ𝓍17 = 2ℰ𝓍16
ℰ𝓍18 = ℰ𝓍1 * ℰ𝓍17
ℰ𝓍19 = conj(ℰ𝓍18)
ℰ𝓍20 = rec[i - 1, j] + ℰ𝓍3
ℰ𝓍21 = ℰ𝓍20 - ℰ𝓍5
ℰ𝓍22 = 2ℰ𝓍21
ℰ𝓍23 = ℰ𝓍1 * ℰ𝓍22
ℰ𝓍24 = conj(ℰ𝓍23)
ℰ𝓍25 = rec[i - 1, j] + ℰ𝓍3
ℰ𝓍26 = ℰ𝓍25 - ℰ𝓍5
ℰ𝓍27 = 2ℰ𝓍26
ℰ𝓍28 = -4ℰ𝓍27
ℰ𝓍29 = ℰ𝓍1 * ℰ𝓍28
ℰ𝓍30 = conj(ℰ𝓍29)
𝛥rec[i - 1, j] = 𝛥rec[i - 1, j] + ℰ𝓍9
𝛥rec[i + 1, j] = 𝛥rec[i + 1, j] + ℰ𝓍14
𝛥rec[i, j + 1] = 𝛥rec[i, j + 1] + ℰ𝓍19
𝛥rec[i, j - 1] = 𝛥rec[i, j - 1] + ℰ𝓍24
𝛥rec[i, j] = 𝛥rec[i, j] + ℰ𝓍30
end
end
end
x = rand(10,10);
dx1 = fill!(similar(x), 0); dx2 = fill!(similar(x), 0);
del = ones(1);
grad!(dx1, del, x); gradavx!(dx2, del, x)
dx1 .- dx2
```

Start with the grad! and gradavx! reproducer in the issue and run it in Julia, comparing dx1 and dx2. Investigate how @avx handles the constant-offset accesses and loop-carried updates; done means the vectorized result matches the non-vectorized result for this case and appropriate coverage exists.

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