Minor refactor of reduction ops

src/pystencils/backend/ast/vector.py

src/pystencils/backend/functions.py

src/pystencils/backend/kernelcreation/freeze.py

src/pystencils/backend/platforms/generic_cpu.py

src/pystencils/backend/platforms/x86.py

src/pystencils/compound_op_mapping.py

src/pystencils/sympyextensions/reduction.py