SDAG: Stop relying on Select's return value in SystemZ's splitLargeImmediate. NFC

The call to Select on Upper here happens in an unusual order in order
to defeat the constant folding that getNode() does. Add a comment
explaining why we can't just move the Select to later to avoid a
Handle, and wrap the call to SelectCode in a handle so we don't need
its return value.

This is part of the work to have Select return void instead of an
SDNode *, which is in turn part of llvm.org/pr26808.

llvm-svn: 268990