[SelectionDAG][X86] Move setcc mask splitting for...

[SelectionDAG][X86] Move setcc mask splitting for mload/mstore/mgather/mscatter from DAGCombiner to the type legalizer.

We may be able to look to how VSELECT is handled to further
improve this, but this appears to be neutral or an improvement
on the test cases we have.

llvm-svn: 368344