add a new SDNPVariadic SDNP node flag, and use it in

add a new SDNPVariadic SDNP node flag, and use it in
dag isel gen instead of instruction properties.  This
allows the oh-so-useful behavior of matching a variadic
non-root node.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@98934 91177308-0d34-0410-b5e6-96231b3b80d8