[X86] Add a further unrolled madd reduction test case that shows several deficiencies.

The AVX2 check lines show two issues. An ADD that became an OR
because we knew the input was disjoint, but really it was zero
so we should have just removed the ADD/OR all together.

Relatedly we use 128-bit VPMADDWD instructions followed by
256-bit VPADDD operations. We should be able to narrow these
VPADDDs.

llvm-svn: 369736