[X86] Add an X86ISD::MSCATTER node for consistency with the X86ISD::MGATHER.

This makes the fact that X86 needs an explicit mask output not part of the type constraint for the ISD::MSCATTER.

This also gives the X86ISD::MGATHER/MSCATTER nodes a common base class simplifying the address selection code in X86ISelDAGToDAG.cpp

llvm-svn: 318823