Vectorization improvements

pystencils/backends/arm_instruction_sets.py

pystencils/backends/cbackend.py

pystencils/backends/ppc_instruction_sets.py

pystencils/backends/simd_instruction_sets.py

pystencils/backends/x86_instruction_sets.py

pystencils/cpu/cpujit.py

pystencils/include/aesni_rand.h

pystencils/include/philox_rand.h

pystencils/include/ppc_altivec_helpers.h

pystencils/llvm/llvmjit.py

pystencils_tests/test_conditional_vec.py

pystencils_tests/test_llvm.py

pystencils_tests/test_random.py

pystencils_tests/test_vectorization.py

pytest.ini