[x86] Clean up and generate detailed FileCheck assertions for
avx-sext.ll using my new script.

Also add an AVX2 mode to this test.

Part of cleaning up the test suite before enabling the new vector
shuffle lowering. This also highlights some of the abysmal failures of
the old shuffle lowering. Check out those 'pinsrw' and 'pextrw'
sequences!
[x86] Generate the FileCheck assertions for avx-blend.ll with my new
script to make them nice and predictable. This will ease updating them
for the new vector shuffle lowering and seeing the delta if any.