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/testData/codegen/box/coroutines/kt52311_nullOnLeft.kt

@@ -0,0 +1,29 @@
+// WITH_STDLIB
+// WITH_COROUTINES
+import helpers.*
+import kotlin.coroutines.*
+import kotlin.coroutines.intrinsics.*
+
+fun someCondition() = true
+
+suspend fun suspendHere() = suspendCoroutineUninterceptedOrReturn {
+    it.resume(Unit)
+    COROUTINE_SUSPENDED
+}
+
+fun expectString(x: String?) = x!!
+
+suspend fun foo(): String {
+    var x: String? = null
+    if (someCondition()) {
+        x = "OK"
+    }
+    suspendHere()
+    return expectString(x)
+}
+
+fun box(): String {
+    var result = "fail"
+    suspend { result = foo() }.startCoroutine(EmptyContinuation)
+    return result
+}

compiler/testData/codegen/box/coroutines/kt52311_nullOnRight.kt

@@ -0,0 +1,29 @@
+// WITH_STDLIB
+// WITH_COROUTINES
+import helpers.*
+import kotlin.coroutines.*
+import kotlin.coroutines.intrinsics.*
+
+fun someCondition() = false
+
+suspend fun suspendHere() = suspendCoroutineUninterceptedOrReturn {
+    it.resume(Unit)
+    COROUTINE_SUSPENDED
+}
+
+fun expectString(x: String?) = x!!
+
+suspend fun foo(): String {
+    var x: String? = "OK"
+    if (someCondition()) {
+        x = null as String?
+    }
+    suspendHere()
+    return expectString(x)
+}
+
+fun box(): String {
+    var result = "fail"
+    suspend { result = foo() }.startCoroutine(EmptyContinuation)
+    return result
+}