handle loop-generating expressions

generating expressions could be transformed to traditional loops to make the following work
```julia
acc = 0
@avx for i in 1:10
acc += sum(i+j for j in 1:5)
end
```
or for `ntuple()`:
```julia
acc = 0
@avx for i in 1:10
acc += sum(ntuple(j->i+j, Val(5)))
end
```
Edit: I guess this is related to handling anonymous functions, but at least the generator expression is directly available in the ast.