Allow intrinsics to be used in place of node matchables.

TableGen has support for using an intrinics name directly in a DAG,
but this breaks down when referring to just a node, as that's
handled initializer list stuff entirely via subclassing in the
parser. That is, using an instrinsic like "(int_my_intrinsic ...)"
works fine. Using it standalone for parameterizing the operator
in such a DAG does not.

Fixing this is simple enough, as we simply declare Intrinsic
as deriving from SDPatternOperator, which is the class name
intended for exactly this purpose in TargetSelectionDAG.td.

When the intrinsic is actually used in the DAG pattern, it will
be recognized and expanded to an intrinsic_wo_chain (et. al.)
just like when it's used directly.

Incoming ARM NEON cleanup based on this and a bit of functionality
improvement after that.

llvm-svn: 159973