[x86] With the stronger canonicalization of shuffles added in r218216,
the new vector shuffle lowering no longer needs to check both symmetric
forms of UNPCK patterns for v4f64.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218217 91177308-0d34-0410-b5e6-96231b3b80d8