K2: generate Unit conversion for indexed assignment at raw FIR stage

#KT-59748 Fixed

compiler/testData/codegen/box/fir/listAssignmentInWhen.kt

@@ -0,0 +1,14 @@
+// ISSUE: KT-59748
+// WITH_STDLIB
+
+fun foo(list: MutableList<Any?>, condition: Boolean): Unit = when {
+    condition -> list[0] = "OK"
+    else -> Unit
+}
+
+fun box(): String {
+    val list = mutableListOf<Any?>("FAIL")
+    foo(list, true)
+    foo(list, false)
+    return list[0] as String
+}