Handing of `end` in indexing

From https://github.com/mcabbott/Tullio.jl/issues/97, this gives an error:
```julia
function col!(out, A)
@avx for i in 1:3
out[i, end] = 10 * A[i]
out[i, begin] = 100 * A[i]
end
out
end
col!(rand(3,3), collect(1:3)) # UndefVarError: end not defined
```

Start by running the Julia reproducer using the @avx loop shown in the issue, then trace how the macro handles indexed expressions containing end and begin. Done means the example expands and runs without UndefVarError while preserving the intended indexing behavior for both forms.

Escrito por el modelo de indexación a partir del texto del issue.