[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!

llvm-svn: 218794