FIR2IR: don't generate unnecessary 'return throw'

#KT-60245 Fixed

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrImplicitCastInserter.kt

@@ -190,7 +190,7 @@ class Fir2IrImplicitCastInserter(
         (data as? IrContainerExpression)?.insertImplicitCasts() ?: data
 
     override fun visitReturnExpression(returnExpression: FirReturnExpression, data: IrElement): IrElement {
-        val irReturn = data as IrReturn
+        val irReturn = data as? IrReturn ?: return data
         val expectedType = returnExpression.target.labeledElement.returnTypeRef
         irReturn.value = irReturn.value.cast(returnExpression.result, returnExpression.result.typeRef, expectedType)
         return data

compiler/fir/fir2ir/src/org/jetbrains/kotlin/fir/backend/Fir2IrVisitor.kt

@@ -498,9 +498,13 @@ class Fir2IrVisitor(
     // ==================================================================================
 
     override fun visitReturnExpression(returnExpression: FirReturnExpression, data: Any?): IrElement {
+        val result = returnExpression.result
+        if (result is FirThrowExpression) {
+            // Note: in FIR we must have 'return' as the last statement
+            return convertToIrExpression(result)
+        }
         val irTarget = conversionScope.returnTarget(returnExpression, declarationStorage)
         return returnExpression.convertWithOffsets { startOffset, endOffset ->
-            val result = returnExpression.result
             // For implicit returns, use the expression endOffset to generate the expected line number for debugging.
             val returnStartOffset = if (returnExpression.source?.kind is KtFakeSourceElementKind.ImplicitReturn) endOffset else startOffset
             IrReturnImpl(