[x86] Add the beginnings of some tests for our v8f32 shuffle lowering
under AVX.

This really just documents the current state of the world. I'm going to
try to flesh it out to cover any test cases I plan to improve prior to
improving them so that the delta made by changes is actually visible to
code reviewers.

This is made easier by the fact that I now have a script to automate the
process of producing test cases including the check lines. =]