Robustify specialised IF/IF conversion introduced in 729ce57
* When unlinking a node from its destination LVAR, always mark
the node as potentially up for dead code elimination. IR2 can
become really confused when converting dead code; a more
systematic cleanup pass might provide a useful safety net.
* The changes make a widely-used ir1-manipulation function safer,
so this might also fix some other obscure compiler bug.
* Reported by James Y Knight on IRC and Fila Kolodny on Launchpad
(fixes lp#1183496).