[Headers][X86] Add -O0 assembly tests for avx2 intrinsics.

We agreed for r245605 that, as long as we don't affect -O0 codegen
too much, it's OK to use native constructs rather than intrinsics.
Let's test that, starting with AVX2 here.

See PR24580.

Differential Revision: http://reviews.llvm.org/D12212

llvm-svn: 245987