IR: remove cast to IrStatement in IrDeclarationBase.transform

Default implementation of `IrStatement.transform` contained a cast to
`IrStatement`. Since almost all IR elements inherit from IrStatement,
this implementation was used in many subclasses. However, checkcast to
interface is slow and this place was indeed noticeable in the profiler
snapshot. Since not that many places really expected to get IrStatement
out of IrStatement.transform, introduce a new method
`transformStatement` that does this cast, and use it in all those
places. Meanwhile, most implementations will use the IrElement's
implementation of `transform` which merely invokes `accept` without
casts.

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

@@ -17,6 +17,7 @@
 package org.jetbrains.kotlin.ir
 
 import org.jetbrains.kotlin.ir.visitors.IrElementTransformer
+import org.jetbrains.kotlin.ir.visitors.IrElementTransformerVoid
 import org.jetbrains.kotlin.ir.visitors.IrElementVisitor
 
 interface IrElement {
@@ -33,10 +34,10 @@ interface IrElement {
     fun <D> transformChildren(transformer: IrElementTransformer<D>, data: D): Unit
 }
 
-interface IrStatement : IrElement {
-    override fun <D> transform(transformer: IrElementTransformer<D>, data: D): IrStatement =
-        super.transform(transformer, data) as IrStatement
-}
+interface IrStatement : IrElement
+
+fun IrStatement.transformStatement(transformer: IrElementTransformerVoid): IrStatement =
+    transform(transformer, null) as IrStatement
 
 inline fun <reified T : IrElement> IrElement.assertCast(): T =
     if (this is T) this else throw AssertionError("Expected ${T::class.simpleName}: $this")

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/declarations/IrDeclaration.kt

@@ -24,7 +24,6 @@ import org.jetbrains.kotlin.ir.IrElementBase
 import org.jetbrains.kotlin.ir.IrStatement
 import org.jetbrains.kotlin.ir.ObsoleteDescriptorBasedAPI
 import org.jetbrains.kotlin.ir.symbols.IrSymbol
-import org.jetbrains.kotlin.ir.visitors.IrElementTransformer
 import org.jetbrains.kotlin.name.Name
 
 interface IrSymbolOwner : IrElement {
@@ -46,10 +45,7 @@ interface IrDeclaration : IrStatement, IrMutableAnnotationContainer {
     val factory: IrFactory
 }
 
-abstract class IrDeclarationBase : IrElementBase(), IrDeclaration {
-    override fun <D> transform(transformer: IrElementTransformer<D>, data: D): IrStatement =
-        accept(transformer, data) as IrStatement
-}
+abstract class IrDeclarationBase : IrElementBase(), IrDeclaration
 
 interface IrSymbolDeclaration<out S : IrSymbol> : IrDeclaration, IrSymbolOwner {
     override val symbol: S

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/expressions/IrBody.kt

@@ -17,6 +17,7 @@
 package org.jetbrains.kotlin.ir.expressions
 
 import org.jetbrains.kotlin.ir.IrElementBase
+import org.jetbrains.kotlin.ir.IrStatement
 import org.jetbrains.kotlin.ir.declarations.IrFactory
 import org.jetbrains.kotlin.ir.visitors.IrElementTransformer
 import org.jetbrains.kotlin.ir.visitors.IrElementVisitor
@@ -58,7 +59,7 @@ abstract class IrBlockBody : IrBody(), IrStatementContainer {
 
     override fun <D> transformChildren(transformer: IrElementTransformer<D>, data: D) {
         statements.forEachIndexed { i, irStatement ->
-            statements[i] = irStatement.transform(transformer, data)
+            statements[i] = irStatement.transform(transformer, data) as IrStatement
         }
     }
 }

compiler/ir/ir.tree/src/org/jetbrains/kotlin/ir/expressions/impl/IrBlockImpl.kt

@@ -97,7 +97,7 @@ class IrReturnableBlockImpl(
 
     override fun <D> transformChildren(transformer: IrElementTransformer<D>, data: D) {
         statements.forEachIndexed { i, irStatement ->
-            statements[i] = irStatement.transform(transformer, data)
+            statements[i] = irStatement.transform(transformer, data) as IrStatement
         }
     }
 }