[x86] Make the new vector shuffle legality test on by default, which

[x86] Make the new vector shuffle legality test on by default, which
reflects the fact that the x86 backend can in fact lower any shuffle you
want it to with reasonably high code quality.

My recent work on the new vector shuffle has made this regress *very*
little. The diff in the test cases makes me very, very happy.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229958 91177308-0d34-0410-b5e6-96231b3b80d8