[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/VarDeferredInitInOpenClass.kt

@@ -1,3 +1,4 @@
+// FIR_IDENTICAL
 // !DIAGNOSTICS: -DEBUG_INFO_LEAKING_THIS
 // a = final + not initialized in place + deferred init
 // e = final + not initialized in place