[x86] Teach the rest of the 'target shuffle' machinery about blends and

add VPBLENDD to the InstPrinter's comment generation so we get nice
comments everywhere.

Now that we have the nice comments, I can see the bug introduced by
a silly typo in the commit that enabled VPBLENDD, and have fixed it. Yay
tests that are easy to inspect.

llvm-svn: 218335