When strict enums are in use, we don't always get ranges reduced 
sufficiently thru casts and such.  Ranger vrp has been missing the odd 
case because I didn't notice that gimple_stmt_nonnegative_warnv_p() use 
to be called on all integral results that were varying, not just calls 
like it was doing. this corrects that.

Bootstrapped on x86_64-pc-linux-gnu with no regressions.  Pushed.

Andrew