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.tree/src/org/jetbrains/kotlin/ir/IrElement.kt

@@ -15,8 +15,7 @@ interface IrElement {
 
     fun <R, D> accept(visitor: IrElementVisitor<R, D>, data: D): R
 
-    fun <D> transform(transformer: IrElementTransformer<D>, data: D): IrElement =
-        accept(transformer, data)
+    fun <D> transform(transformer: IrElementTransformer<D>, data: D): IrElement
 
     fun <D> acceptChildren(visitor: IrElementVisitor<Unit, D>, data: D)
 

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/IrElementBase.kt

@@ -16,4 +16,9 @@
 
 package org.jetbrains.kotlin.ir
 
-abstract class IrElementBase : IrElement
+import org.jetbrains.kotlin.ir.visitors.IrElementTransformer
+
+abstract class IrElementBase : IrElement {
+    override fun <D> transform(transformer: IrElementTransformer<D>, data: D): IrElement =
+        accept(transformer, data)
+}