[x86] Fix PR22377, a regression with the new vector shuffle legality

[x86] Fix PR22377, a regression with the new vector shuffle legality
test.

This was just a matter of the DAG combine for vector shuffles being too
aggressive. This is a bit of a grey area, but I think generally if we
can re-use intermediate shuffles, we should. Certainly, given the test
cases I have available, this seems like the right call.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229285 91177308-0d34-0410-b5e6-96231b3b80d8