[X86] Add some reduction add test cases that show sub-optimal code on avx2 and later.

For v4i8 and v8i8 when the reduction starts with a load we end up
shifting the data in the scalar domain and copying to the vector
domain a second time using a broadcast.

We already copied it to the vector domain once. It's better to
just shuffle it there.

llvm-svn: 368544