The real maximum was GPU_SAMPLER_ICON, not GPU_SAMPLER_REPEAT, my
bad. rBa31a87f8943a: T81340: UBSan: load of value .. not valid for GPU enum type

Move GPU_SAMPLER_MAX out of the enum since it's used as an int
at many places.
Also, the macro ENUM_OPERATORS needs a maximum, and this enumerator
cannot be used as the argument of that macro. It creates wrong values
after ~ operator.