Deprecate prevind(::Any, ::Integer)

See discussion in #9209 

It seems likely that users that trigger the deprecation rather would have a `MethodError`, and the suggestion to use `i-1` instead is probably bad advise. What do people think?
`prevind` is to get the previous valid index. Getting the previous nonzero index would be a different function in my mind.

Anyway, `AbstractArray` isn't wrong, so I'll change it if nobody has more serious concerns than me.