[X86][SSE] Add support for <64 x i1> bool reduction

This generalizes the existing <32 x i1> pre-AVX2 split code to support reductions from <64 x i1> as well, we can probably generalize to any larger pow2 case in the future if the (unlikely) need ever arises.

We still need to tweak combineBitcastvxi1 to improve AVX512F codegen as its assumes vXi1 types should be handled on the mask registers even when they aren't legal.

Differential Revision: https://reviews.llvm.org/D67070

llvm-svn: 371328