IR: Fix scope transparency in ReturnableBlockLowering

An IrReturnableBlock introduces a new variable scope and shouldn't be
replaced with a transparent IrComposite block.

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

@@ -127,7 +127,7 @@ class ReturnableBlockTransformer(val context: CommonBackendContext, val containe
         returnMap.remove(expression.symbol)
 
         if (!hasReturned) {
-            return IrCompositeImpl(
+            return IrBlockImpl(
                 expression.startOffset,
                 expression.endOffset,
                 expression.type,