Support deserialized IR in IfNullExpressionsFusionLowering

Deserializer generates simple branches instead of IrElseBranches.

compiler/ir/backend.common/src/org/jetbrains/kotlin/backend/common/lower/IfNullExpressionsFusionLowering.kt

@@ -172,11 +172,22 @@ class IfNullExpressionsFusionLowering(val context: CommonBackendContext) : FileL
         val arg1 = condition0.getValueArgument(1) as? IrConst<*> ?: return null
         if (arg1.value != null) return null
 
-        val branch1 = whenExpr.branches[1] as? IrElseBranch ?: return null
+        val elseResult = whenExpr.branches[1].getElseBranchResultOrNull() ?: return null
 
-        return IfNullExpr(whenExpr.type, subjectVar, branch0.result, branch1.result)
+        return IfNullExpr(whenExpr.type, subjectVar, branch0.result, elseResult)
     }
 
+    private fun IrBranch.getElseBranchResultOrNull() =
+        when {
+            this is IrElseBranch ->
+                result
+            // deserialization generates a simple branch with const 'true' condition
+            condition is IrConst<*> && condition.value == true ->
+                result
+            else ->
+                null
+        }
+
     private fun IrExpression.isTrivial() =
         this is IrExpressionWithCopy