[x86] Fix the v16i16 blend logic I added in the prior commit and add the

missing test cases for it.

Unsurprisingly, without test cases, there were bugs here. Surprisingly,
this bug wasn't caught at compile time. Yep, there is an X86ISD::BLENDV.
It isn't wired to anything. Oops. I'll fix than next.

llvm-svn: 218434