[x86] Fix PR21139, one of the last remaining regressions found in the
new vector shuffle lowering.

This is loosely based on a patch by Marius Wachtler to the PR (thanks!).
I refactored it a bi to use std::count_if and a mutable array ref but
the core idea was exactly right. I also added some direct testing of
this case.

I believe PR21137 is now the only remaining regression.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219081 91177308-0d34-0410-b5e6-96231b3b80d8