* module/language/cps/cse.scm (elide-predecessor, prune-branch)
(prune-successors, term-successors): New helpers.
(eliminate-common-subexpressions-in-fun): When we modify the CFG,
update the analysis. Also, thread the substs map through CSE so that
closures in high-level CPS can take advantage of eliminated variables.
(fold-renumbered-functions): Take multiple seeds.
(eliminate-common-subexpressions): Thread var substs map through CSE.