Add IDEA data-flow analysis to guess nullability

Add "if return..." folding to "return if"

j2k/testData/multiFile/NullabilityByDFa/SomeServiceUsage.kt

@@ -0,0 +1,47 @@
+class SomeServiceUsage {
+
+    val service: SomeService
+        get() = SomeService.getInstanceNotNull()
+
+    val serviceNullable: SomeService?
+        get() = SomeService.getInstanceNullable()
+
+    // elvis
+    val serviceNotNullByDataFlow: SomeService
+        get() {
+            val s = SomeService.getInstanceNullable()
+            return s ?: SomeService.getInstanceNotNull()
+        }
+
+    // nullable, bang-bang
+    fun aString1(): String? {
+        return serviceNullable!!.nullableString()
+    }
+
+    // nullable
+    fun aString2(): String? {
+        return service.nullableString()
+    }
+
+    // not nullable
+    fun aString3(): String {
+        return service.notNullString()
+    }
+
+    // nullable, no bang-bang
+    fun aString4(): String? {
+        return serviceNotNullByDataFlow.nullableString()
+    }
+
+    // not nullable, no bang-bang
+    fun aString5(): String {
+        return serviceNotNullByDataFlow.notNullString()
+    }
+
+    // nullable, safe-call
+    fun aString6(): String? {
+        val s = serviceNullable
+        return s?.nullableString()
+    }
+
+}