* module/language/cps/cse.scm (forward-cont, forward-branch)
(compute-avail-and-bool-edge): New helpers.
(add-equivalent-expression!): Allow idempotent adds; can happen now
when revisiting a cont after changes to its predecessors.
(fold-branch): New helper.
(eliminate-common-subexpressions-in-fun): Allow for reductions to
branch predecessors. In that case, revisit the branch, as the CFG
will have changed.