Support for 'triangular' loops

Suppose I have a function
```julia
function pairwise(a)
ret = Matrix{eltype(a)}(undef, length(a), length(a))
for i in 1:(length(a)-1)
for j in (i+1):length(a)
ret[i, j] = a[i] * a[j]
end
end
UpperTriangular(ret)
end
```
Is there a way to represent this loop over the 'upper triangular indices' of `ret` with LoopVectorization.jl?

Start by reading LoopVectorization.jl’s existing loop syntax and trying the provided pairwise example with its upper-triangular indices. Determine whether triangular-loop representation is supported and define completion as making this example expressible and vectorizable, with coverage for the resulting upper-triangular iteration.

索引モデルが issue の本文から書いたものです。