[x86] Teach the new vector shuffle lowering to lower v4i64 vector
shuffles using the AVX2 instructions. This is the first step of cutting
in real AVX2 support.

Note that I have spotted at least one bug in the test cases already, but
I suspect it was already present and just is getting surfaced. Will
investigate next.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218338 91177308-0d34-0410-b5e6-96231b3b80d8