My recent patch to add additional vector lengths didn't address the 
vector reductions yet.

This patch adds the missing support. Shorter vectors use fewer reduction 
steps, and the means to extract the final value has been adjusted.

Lacking from this is any useful costs, so for loops the vect pass will 
almost always continue to choose the most expensive 64-lane vectors, and 
therefore reductions, with masking for smaller sizes. This works, but 
probably could be improved.

For SLP the compiler will usually use a more appropriately sized 
reduction, so this patch is useful for that case.

Andrew