re-allow implicit prevfloat for signed step sizes
          #59836
        
          
      
                
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#59668 changed
prevfloat(x, d)to no longer just callnextfloat(x, -d), so as to avoid bugs resulting from intermediate overflow whendis unsigned.as predicted in #59668 (comment) and then observed in #59668 (comment), this causes some package failures. The failures are exposing a bug, so it's ultimately a good thing, but this PR would restore the old behavior at least when
d isa Signedand only fails when we'd actually hit the unsigned buggy case.