[x86] Now that the new vector shuffle legality is enabled and everything

[x86] Now that the new vector shuffle legality is enabled and everything
is going well, remove the flag and the code for the old legality tests.

This is the first step toward removing the entire old vector shuffle
lowering. *Much* more code to delete coming up next.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229963 91177308-0d34-0410-b5e6-96231b3b80d8