[NI] Soften restictions on using Nothing as proper constraint for full call completion

Consider lower `Nothing` constraint non-proper only if there is a dependant not analyzed postponed atom.
Early completion to `Nothing` provides data flow info for smart casts.

KT-35668 Fixed

compiler/testData/diagnostics/tests/controlFlowAnalysis/elvisNotProcessed.kt

@@ -15,7 +15,7 @@ val ww = if (true) {
     <!OI;TYPE_MISMATCH!>{ true }<!> <!USELESS_ELVIS!>?: null!!<!>
 }
 else if (true) {
-    <!NI;TYPE_MISMATCH, NI;TYPE_MISMATCH, TYPE_MISMATCH!>{ <!NI;CONSTANT_EXPECTED_TYPE_MISMATCH!>true<!> }<!> <!USELESS_ELVIS!>?: null!!<!>
+    <!OI;TYPE_MISMATCH!>{ true }<!> <!USELESS_ELVIS!>?: null!!<!>
 }
 else {
     null!!