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
```