[x86] Fix yet another issue with widening vector shuffle elements.
I spotted this by inspection when debugging something else, so I have no
test case what-so-ever, and am not even sure it is possible to
realistically trigger the bug. But this is what was intended here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218565 91177308-0d34-0410-b5e6-96231b3b80d8