[FIR] fix false negative MUST_BE_INITIALIZED for deferred init + custom setter case

^KT-58346 Fixed
Review: https://jetbrains.team/p/kt/reviews/9967

BTW this commit accidentaly and partially fixes KT-57553 for K2, because
of a cleaner K2 architecture. I will unify MUST_BE_INITIALIZED behaviour
in K1 and K2 in the next commits

compiler/testData/diagnostics/tests/backingField/InitCustomSetter.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 class My(val v: Int) {
     // Ok: setter is just private
     var x: Int