[X86] Lower all ISD::MGATHER nodes to X86ISD:MGATHER.

Now we consistently represent the mask result without relying on isel ignoring it.

We now have a more general SDNode and type constraints to represent these nodes in isel patterns. This allows us to present both both vXi1 and XMM/YMM mask types with a single set of constraints.

llvm-svn: 318821