[X86] Add missing check for 'isINSERTPSMask' in method 'isShuffleMaskLegal'.

This helps the DAGCombiner to identify more opportunities to fold shuffles.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221684 91177308-0d34-0410-b5e6-96231b3b80d8