Implicit exhaustive when check for definite variable initialization (KT-8700)

compiler/testData/diagnostics/tests/when/BranchFalseBypass.kt

@@ -3,10 +3,10 @@ enum class My { A, B }
 fun test(a: My): String {
     val q: String?
 
-    when (a) {
+    <!DEBUG_INFO_IMPLICIT_EXHAUSTIVE!>when (a) {
         My.A -> q = "1"
         My.B -> q = "2"
-    }
+    }<!>
     // When is exhaustive
     return <!DEBUG_INFO_SMARTCAST!>q<!>
 }