Properties without accessors are generated as

IrGetBackingField / IrSetBackingField expressions.

compiler/testData/ir/irText/expressions/setFieldWithImplicitCast.txt

@@ -0,0 +1,16 @@
+FILE /Derived.kt
+  CLASS CLASS Derived
+    CONSTRUCTOR public constructor Derived()
+      BLOCK_BODY
+        DELEGATING_CONSTRUCTOR_CALL Base
+        INSTANCE_INITIALIZER_CALL classDescriptor=Derived
+    FUN public final fun setValue(/*0*/ v: kotlin.Any): kotlin.Unit
+      BLOCK_BODY
+        WHEN type=kotlin.Unit operator=IF
+          if: TYPE_OP operator=INSTANCEOF typeOperand=kotlin.String
+            GET_VAR v type=kotlin.Any operator=null
+          then: BLOCK type=kotlin.Unit operator=null
+            SET_BACKING_FIELD value type=kotlin.Unit operator=EQ
+              receiver: THIS public final class Derived : Base type=Derived
+              value: TYPE_OP operator=IMPLICIT_CAST typeOperand=kotlin.String!
+                GET_VAR v type=kotlin.Any operator=null