Do not introduce new shuffle patterns after operation legalization if SHUFFLE_VECTOR
was marked custom.  The target independent DAG combine has no way to know if
the shuffles it is introducing are ones that the target could support or not.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216678 91177308-0d34-0410-b5e6-96231b3b80d8