Fix the case of safe call and elvis operators: flexible types may be nullable

compiler/testData/diagnostics/tests/nullableTypes/safeCallOnTypeWithNullableUpperBound.kt

@@ -1,11 +1,11 @@
-fun <T> test(t: T): String {
+fun <T> test(t: T): String? {
     if (t != null) {
         return t<!UNNECESSARY_SAFE_CALL!>?.<!>toString()
     }
     return t?.toString()
 }
 
-fun <T> T.testThis(): String {
+fun <T> T.testThis(): String? {
     if (this != null) {
         return this<!UNNECESSARY_SAFE_CALL!>?.<!>toString()
     }

compiler/testData/diagnostics/tests/platformTypes/elvis.kt

@@ -0,0 +1,14 @@
+// FILE: p/J.java
+
+package p;
+
+public class J {
+    public String s() { return null; }
+}
+
+// FILE: k.kt
+import p.*
+
+fun test(j: J) {
+    j.s()?.length ?: ""
+}

compiler/testData/diagnostics/tests/platformTypes/safeCall.kt

@@ -0,0 +1,15 @@
+// !CHECK_TYPE
+// FILE: p/J.java
+
+package p;
+
+public class J {
+    public String s() { return null; }
+}
+
+// FILE: k.kt
+import p.*
+
+fun test(j: J) {
+    j.s()?.length.checkType { it : _<Int?> }
+}