IR: move IrElement.transform implementation to IrElementBase

To avoid extra indirection via `IrElement$DefaultImpls` for elements
whose `transform` uses the default implementation. And also, to simplify
migration to auto-generated IR tree.

compiler/ir/ir.ir2cfg/src/org/jetbrains/kotlin/ir2cfg/nodes/MergeCfgElement.kt

@@ -27,9 +27,11 @@ class MergeCfgElement(val from: IrElement, val name: String) : CfgIrElement {
 
     override fun <R, D> accept(visitor: IrElementVisitor<R, D>, data: D) = visitor.visitElement(this, data)
 
+    override fun <D> transform(transformer: IrElementTransformer<D>, data: D): IrElement = accept(transformer, data)
+
     override fun <D> acceptChildren(visitor: IrElementVisitor<Unit, D>, data: D) = Unit
 
     override fun <D> transformChildren(transformer: IrElementTransformer<D>, data: D) = Unit
 
     override fun toString() = "$name: ${from.dump()}"
-}
+}