Add regression tests for obsolete issues

#KT-50909
 #KT-50974
 #KT-51888

compiler/testData/codegen/box/inlineClasses/kt50909.kt

@@ -0,0 +1,17 @@
+// TARGET_BACKEND: JVM_IR
+// WITH_STDLIB
+
+data class Parent(val child: Parent.Child?) {
+    val result =
+        if (this.child == null) foo(this.child)
+        else "Fail"
+
+    @JvmInline
+    value class Child(val value: String)
+}
+
+fun foo(x: String?): String =
+    x ?: "OK"
+
+fun box(): String =
+    Parent(null).result