[X86][AVX512BW] Add support for v64i8 multiplies

Extend the existing lowering of vXi8 multiplies to support v64i8 on avx512bw targets.

I added the Lower512IntArith helper function to help with this - not sure how often this could be used in the future, but it seemed better than putting all that logic inside LowerMUL.

Differential Revision: http://reviews.llvm.org/D18937

llvm-svn: 265902