JVM: correctly merge typed null values

1. merge(null of type A, null of type B) = null of unknown type;
2. merge(null of type A, something of type B) = merge(unknown null, B).

^KT-52311 Fixed

compiler/backend/src/org/jetbrains/kotlin/codegen/coroutines/SpilledVariableFieldTypesAnalysis.kt

@@ -161,7 +161,7 @@ private fun Type.isIntLike(): Boolean = when (sort) {
 }
 
 // Represents [ACONST_NULL, CHECKCAST Type] sequence result.
-internal class TypedNullValue(type: Type) : BasicValue(type)
+internal class TypedNullValue(type: Type) : StrictBasicValue(type)
 
 // Preserves nulls through CHECKCASTS.
 private class NullCheckcastAwareOptimizationBasicInterpreter : OptimizationBasicInterpreter() {
@@ -171,4 +171,12 @@ private class NullCheckcastAwareOptimizationBasicInterpreter : OptimizationBasic
         }
         return super.unaryOperation(insn, value)
     }
-}
+
+    override fun merge(v: BasicValue, w: BasicValue): BasicValue =
+        when {
+            v is TypedNullValue && w is TypedNullValue -> if (v.type == w.type) v else StrictBasicValue.NULL_VALUE
+            v is TypedNullValue -> super.merge(StrictBasicValue.NULL_VALUE, w)
+            w is TypedNullValue -> super.merge(v, StrictBasicValue.NULL_VALUE)
+            else -> super.merge(v, w)
+        }
+}