Generalize the SDOperand->SDOperand form of

SelectionDAG::ReplaceAllUsesWith to handle replacement of
an SDOperand with *any* sdoperand, not just one for a node with
a single result.  Note that this has a horrible FIXME'd hack in it
to work around PR1975.  This should be removed when PR1975 is fixed.

llvm-svn: 46674