Branch predictor hint for exceptional branch
Repeat newSViv optimization for newSVuv

Pretty much the same change as
bd30fe8921c88e4677c2279b442a56a11ae037b4 was for newSViv.
Attempt to bring newSViv/uv optimization to newRV

Cf. bd30fe8921c88e4677c2279b442a56a11ae037b4 for details.