guest_ppc_toIR: Call vpanic not just vex_printf when the impossible happens

guest_ppc_toIR: Call vpanic not just vex_printf when the impossible happens

is_Zero_Vector, is_Denorm_Vector, is_NaN_Vector and negate_Vector
only handle an Ity_I32 element size. And that is also what they are
currently being called with. In case they would ever be called with
a different element_size they would simply vex_printf and continue
(producing bogus/impossible results). To make this a bit more future
proof (and to silence a static analyzer) vpanic instead.