[IR] Add test describes case from KT-48912

It actually doesn't check behaviour because we don't have proper infra
for that but explains a bit a problem for further investigations.

compiler/testData/codegen/box/ir/serializationRegressions/anonFakeOverride.kt

@@ -0,0 +1,59 @@
+// KJS_WITH_FULL_RUNTIME
+// WITH_RUNTIME
+
+// extracted case from KT-48912
+
+// MODULE: libxxxxx
+// FILE: l1.kt
+
+package com.soywiz.korag.shader
+
+open class Operand
+
+open class Variable : Operand()
+
+class Program {
+    class Stm(val o: Operand)
+
+    open class Visitor<E>(val default: E) {
+        open fun visit(stm: Stm?) {
+            if (stm?.o is Operand) visit(stm.o)
+        }
+        open fun visit(o: Operand): E {
+            if (o is Uniform) visit(o)
+            return default
+        }
+        open fun visit(operand: Uniform): E = default
+    }
+}
+
+open class Uniform(val result: String): Variable()
+
+open class Shader(val stm: Program.Stm?) {
+
+    // Here for object.visit(Program.Stm?) FO were 2 different mangles
+    // 1.5.31: <anonymous>#static(kotlin.collections.LinkedHashSet<com.soywiz.korag.shader.Uniform>){}visit(com.soywiz.korag.shader.Program.Stm?){}
+    // 1.6.0+: {}uniforms<anonymous>#static(kotlin.collections.LinkedHashSet<com.soywiz.korag.shader.Uniform>){}visit(com.soywiz.korag.shader.Program.Stm?){}
+    // So fix is to make sure those mangles are identical.
+
+    val uniforms = LinkedHashSet<Uniform>().also { out ->
+        object : Program.Visitor<Unit>(Unit) {
+            override fun visit(uniform: Uniform) {
+                out.add(uniform)
+            }
+        }.visit(stm)
+    }.toSet()
+}
+
+// MODULE: main(libxxxxx)
+// FILE: l2.kt
+
+import com.soywiz.korag.shader.*
+
+class D : Shader(Program.Stm((Uniform("OK"))))
+
+fun foo(): Shader = D()
+
+fun box(): String {
+    return foo().uniforms.first().result
+}