[x86] Teach both sext and zext vector tests to cover a nice wide range
of architectures: SSE2, SSSE3, SSE4.1, AVX, and AVX2.

Unfortunately, this exposses the absolute horror of the code we generate
for many of these patterns. Anyone wanting to familiarize themselves
with the x86 backend and improve performance could do a lot of good
sitting down and making these test cases not look so terrible. While the
new vector shuffle code I'm working on well help some, it won't fix all
of the crimes here.