[FIR] Properly support smartcasts on stable when subjects in when conditions

^KT-49860 Fixed

compiler/testData/diagnostics/tests/when/DuplicatedLabels.fir.kt

@@ -40,7 +40,7 @@ fun third(arg: Any?): Int {
         is String -> return 0
         is Double -> return 1
         is <!DUPLICATE_LABEL_IN_WHEN!>Double<!> -> return 2
-        <!DUPLICATE_LABEL_IN_WHEN!>null<!> -> return 3
+        <!DUPLICATE_LABEL_IN_WHEN, SENSELESS_NULL_IN_WHEN!>null<!> -> return 3
         !is String -> return 4
         else -> return 5
     }

compiler/testData/diagnostics/testsWithStdLib/contracts/smartcasts/when/kt36818.fir.kt

@@ -1,7 +1,7 @@
 fun main(x1: Double?, range: ClosedRange<Double>) {
     when (x1) {
         null -> throw Exception()
-        <!ARGUMENT_TYPE_MISMATCH!>in range<!> -> {} // error, no smartcast from previous branch, OK in OI
+        in range -> {} // error, no smartcast from previous branch, OK in OI
     }
 
     when {