[LegalizeDAG] Move legalization of scatter and masked store from LegalizeVectorOps to LegalizeDAG.

This is where we legalize gather and masked load so this is consistent.

Since these ops are always on vectors I've chosen to go with LegalizeDAG since that's what we do for other vector only ops like BUILD_VECTOR, VECTOR_SHUFFLE, etc. The ScalarizeMaskedMemIntrinsic pass should take care of scalarizing these before SelectionDAG so hopefully we don't need to worry about illegally typed scalar ops being emitted in the legalizing. If we did we would need to do this in LegalizeVectorOps so we could get the second type legalization that runs between LegalizeVectorOps and LegalizeDAG.

llvm-svn: 343947