Escape Analysis Specifically for Slices

[ejmeitz]

Right now @analyze_lifetimes tracks slices as well as other temporaries. However, it assumes un-fused broadcasting and will probably break things if used on a fused broadcast.

I think it would be good to have a more specific escape analysis pass that just looks at array slices as these can be contained in broadcast expressions.

For example,

@gc_slices begin
    a[1:5] .= b[1:5] .- c[1:5]
end

would eagerly collect only the slice objects.

To implement this I think we could have the @analyze_lifetimes code just call this new macro as one of its passes.