Add missing operator for avx256::mask_v

This is an attempt to fix the x86_64_v4+vecwid256-centos7-gcc10-opt build which was "broken" by Phys!934 (merged):

../Phys/ParticleCombiners/include/CombKernel/ThOrCombiner.h:176:21: error: no match for 'operator|' (operand types are 'mask_t' {aka 'SIMDWrapper::avx256::mask_v'} and 'SIMDWrapper::avx256::mask_v')

I don't know what I'm doing so this might be wrong... And other overloads are likely missing (but no present build uses them so not a "problem" now).

/cc @mramospe