pr 6644 optimizations

[Stone, Joshua I]

Frank Ch. Eigler wrote:
> Thanks for the pr 6644 code.  Could you investigate though whether all
> those new dynamic_cast<>s could be replaced by more properly typed new
> members in the custom visitors?

Sure - I just pushed a changed that eliminated all but one cast.

I could remove the last one with a function like the existing
expression_is_arrayindex(), but IMO that's just a poor man's
dynamic_cast<>.

Josh