[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218434 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Remove the defunct X86ISD::BLENDV entry -- we use vector selects
for this now.

Should prevent folks from running afoul of this and not knowing why
their code won't instruction select the way I just did...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218436 91177308-0d34-0410-b5e6-96231b3b80d8