With:

    open class Root {}
    class Subclass1: Root() {}

    fun typeTests(x: Root, y: Subclass1) {
        val y1: Subclass1 = if (x is Subclass1) { x } else { y }
    }

we now get a slightly different AST, which means we no longer need to
insert a StmtExpr:

     BRANCH
       if: TYPE_OP type=kotlin.Boolean origin=INSTANCEOF typeOperand=<root>.Subclass1
         GET_VAR 'x: <root>.Root declared in <root>.typeTests' type=<root>.Root origin=null
-      then: TYPE_OP type=<root>.Subclass1 origin=IMPLICIT_CAST typeOperand=<root>.Subclass1
-        BLOCK type=<root>.Root origin=null
+      then: BLOCK type=<root>.Subclass1 origin=null
+        TYPE_OP type=<root>.Subclass1 origin=IMPLICIT_CAST typeOperand=<root>.Subclass1
           GET_VAR 'x: <root>.Root declared in <root>.typeTests' type=<root>.Root origin=null

For

    if(r != null) {
        val r2: Rectangle = r

in Kotlin 2 mode, there is no IMPLICIT_NOTNULL check in Kotlin 2 mode:

    then: BLOCK type=kotlin.Unit origin=null
      VAR name:r2 type:java.awt.Rectangle [val]
-       TYPE_OP type=java.awt.Rectangle origin=IMPLICIT_NOTNULL typeOperand=java.awt.Rectangle
-         GET_VAR 'val r: @[FlexibleNullability] java.awt.Rectangle? [val] declared in <root>.foo' type=@[FlexibleNullability] java.awt.Rectangle? origin=null
+       GET_VAR 'val r: @[FlexibleNullability] java.awt.Rectangle? [val] declared in <root>.foo' type=@[FlexibleNullability] java.awt.Rectangle? origin=null
      VAR name:height type:kotlin.Int [val]
        GET_FIELD 'FIELD IR_EXTERNAL_JAVA_DECLARATION_STUB name:height type:kotlin.Int visibility:public' type=kotlin.Int origin=null
          receiver: GET_VAR 'val r2: java.awt.Rectangle [val] declared in <root>.foo' type=java.awt.Rectangle origin=null