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/backend.common/src/org/jetbrains/kotlin/backend/common/lower/InlineClassDeclarationLowering.kt

@@ -15,6 +15,7 @@ import org.jetbrains.kotlin.ir.UNDEFINED_OFFSET
 import org.jetbrains.kotlin.ir.builders.*
 import org.jetbrains.kotlin.ir.declarations.*
 import org.jetbrains.kotlin.ir.expressions.*
+import org.jetbrains.kotlin.ir.transformStatement
 import org.jetbrains.kotlin.ir.util.*
 import org.jetbrains.kotlin.ir.visitors.IrElementTransformerVoid
 import org.jetbrains.kotlin.ir.visitors.transformChildrenVoid
@@ -83,7 +84,7 @@ class InlineClassLowering(val context: CommonBackendContext) {
                     }
 
                     (irConstructor.body as IrBlockBody).statements.forEach { statement ->
-                        +statement.transform(object : IrElementTransformerVoid() {
+                        +statement.transformStatement(object : IrElementTransformerVoid() {
                             override fun visitDelegatingConstructorCall(expression: IrDelegatingConstructorCall): IrExpression {
                                 expression.transformChildrenVoid()
                                 return irBlock(expression) {
@@ -124,7 +125,7 @@ class InlineClassLowering(val context: CommonBackendContext) {
                                 return expression
                             }
 
-                        }, null)
+                        })
                     }
                     +irReturn(irGet(thisVar))
                 }.statements

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

@@ -12,10 +12,7 @@ import org.jetbrains.kotlin.descriptors.Modality
 import org.jetbrains.kotlin.descriptors.ReceiverParameterDescriptor
 import org.jetbrains.kotlin.descriptors.Visibilities
 import org.jetbrains.kotlin.descriptors.Visibility
-import org.jetbrains.kotlin.ir.IrElement
-import org.jetbrains.kotlin.ir.IrStatement
-import org.jetbrains.kotlin.ir.ObsoleteDescriptorBasedAPI
-import org.jetbrains.kotlin.ir.UNDEFINED_OFFSET
+import org.jetbrains.kotlin.ir.*
 import org.jetbrains.kotlin.ir.builders.Scope
 import org.jetbrains.kotlin.ir.builders.declarations.buildConstructor
 import org.jetbrains.kotlin.ir.builders.declarations.buildField
@@ -267,9 +264,9 @@ class LocalDeclarationsLowering(
 
 
         private inner class FunctionBodiesRewriter(val localContext: LocalContext?) : IrElementTransformerVoid() {
-            override fun visitLocalDelegatedProperty(declaration: IrLocalDelegatedProperty) =
+            override fun visitLocalDelegatedProperty(declaration: IrLocalDelegatedProperty): IrStatement =
                 // Both accessors extracted as closures.
-                declaration.delegate.transform(this, null)
+                declaration.delegate.transformStatement(this)
 
             override fun visitClass(declaration: IrClass) = if (declaration in localClasses) {
                 localClasses[declaration]!!.declaration

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

@@ -17,6 +17,7 @@ import org.jetbrains.kotlin.ir.expressions.impl.IrCompositeImpl
 import org.jetbrains.kotlin.ir.expressions.impl.IrDoWhileLoopImpl
 import org.jetbrains.kotlin.ir.symbols.IrReturnableBlockSymbol
 import org.jetbrains.kotlin.ir.symbols.IrSymbol
+import org.jetbrains.kotlin.ir.transformStatement
 
 /**
  * Replaces returnable blocks and `return`'s with loops and `break`'s correspondingly.
@@ -118,7 +119,7 @@ class ReturnableBlockTransformer(val context: CommonBackendContext, val containe
                     builder.irSetVar(variable.symbol, s.value)
                 }
             } else {
-                s.transform(this, null)
+                s.transformStatement(this)
             }
         }
 

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

@@ -26,6 +26,7 @@ import org.jetbrains.kotlin.ir.declarations.*
 import org.jetbrains.kotlin.ir.expressions.*
 import org.jetbrains.kotlin.ir.expressions.impl.IrGetValueImpl
 import org.jetbrains.kotlin.ir.symbols.IrValueParameterSymbol
+import org.jetbrains.kotlin.ir.transformStatement
 import org.jetbrains.kotlin.ir.util.explicitParameters
 import org.jetbrains.kotlin.ir.util.getArgumentsWithIr
 import org.jetbrains.kotlin.ir.visitors.IrElementTransformerVoid
@@ -100,7 +101,7 @@ private fun lowerTailRecursionCalls(context: BackendContext, irFunction: IrFunct
                 )
 
                 oldBodyStatements.forEach {
-                    +it.transform(transformer, null)
+                    +it.transformStatement(transformer)
                 }
 
                 +irBreak(loop)