IR: optimize transformation of declarations/statements lists in-place

Also, inherit IrDeclarationBase from IrElementBase to be able to use a
cast to IrElementBase, which is faster than a cast to IrElement or
IrDeclaration.

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/lower/loops/HeaderProcessor.kt

@@ -579,7 +579,7 @@ internal class IterableLoopHeader(
         loopVariable: IrVariable?,
         loopVariableComponents: Map<Int, IrVariable>,
         builder: DeclarationIrBuilder
-    ) =
+    ): List<IrStatement> =
         with(builder) {
             // loopVariable = iteratorVar.next()
             val iteratorClass = headerInfo.iteratorVariable.type.getClass()!!