Remove SDNode's virtual destructor. This makes it impossible for

SDNode subclasses to keep state that requires non-trivial
destructors, however it was already effectively impossible,
since the destructor isn't actually ever called. There currently
aren't any SDNode subclasses affected by this, and in general
it's desireable to keep SDNode objects light-weight.

This eliminates the last virtual member function in the SDNode
class, so it eliminates the need for a vtable pointer, making
SDNode smaller.

llvm-svn: 62539