Avoid some left-shifts of negative constants

[Paul Eggert]

On 2/7/19 9:14 AM, Joseph Myers wrote:
> If the code is doing left shifts of
> negative values, that may well indicate an unsigned type would have made
> the intended semantics clearer, even if a signed type does work in that
> code.

Although that might be true elsewhere, there is a reasonable argument 
that the code is clearer as-is for this particular case, as it's more 
robust in the presence of integer-width changes. I'd like to see cases 
where the code is obviously clearer after being rewritten to use 
unsigned shifts, before conceding that it's worthwhile to spend time to 
pacify -Wshift-negative-value.