Add change note

Author: owen-mc

java/ql/lib/change-notes/2024-11-14-control-flow-graph-lightweight-IR-layer.md

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+---
+category: breaking
+---
+* The class `ControlFlowNode` (and by extension `BasicBlock`) is no longer
+  directly equatable to `Expr` and `Stmt`. Any queries that have been
+  exploiting these equalities, for example by using casts, will need minor
+  updates in order to fix any compilation errors. Conversions can be inserted
+  in either direction depending on what is most convenient. Available
+  conversions include `Expr.getControlFlowNode()`, `Stmt.getControlFlowNode()`,
+  `ControlFlowNode.asExpr()`, `ControlFlowNode.asStmt()`, and
+  `ControlFlowNode.asCall()`. Exit nodes were until now modelled as a
+  `ControlFlowNode` equal to its enclosing `Callable`; these are now instead
+  modelled by the class `ControlFlow::ExitNode`.