Add IDEA data-flow analysis to guess nullability

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

j2k/testData/fileOrElement/nullability/MethodReturnsNull.kt

@@ -1,9 +1,9 @@
 internal class C {
     fun foo(b: Boolean): String? {
-        if (b) {
-            return "abc"
+        return if (b) {
+            "abc"
         } else {
-            return null
+            null
         }
     }
 }

j2k/testData/fileOrElement/nullability/NullableVariableDotAccess.kt

@@ -1,5 +1,4 @@
 fun foo(s: String?, b: Boolean): Int {
     if (s == null) println("null")
-    if (b) return s!!.length
-    return 10
+    return if (b) s!!.length else 10
 }

j2k/testData/fileOrElement/nullability/Overrides.kt

@@ -25,7 +25,7 @@ internal class C : Base(), I {
         return ""
     }
 
-    override fun zoo(o: Any?): String? {
+    override fun zoo(o: Any?): String {
         return ""
     }
 }

j2k/testData/fileOrElement/nullability/autoNotNull.kt

@@ -1,8 +1,5 @@
 internal class A {
     fun foo(s: String?): Int {
-        if (s != null) {
-            return s.length
-        }
-        return -1
+        return s?.length ?: -1
     }
 }

j2k/testData/fileOrElement/nullability/nullableField.kt

@@ -1,4 +1,3 @@
-// ERROR: Type mismatch: inferred type is String? but String was expected
 // ERROR: Type mismatch: inferred type is Int? but Int was expected
 // ERROR: Type inference failed. Please try to specify type arguments explicitly.
 // ERROR: Using 'remove(Int): T' is an error. Use removeAt(index) instead.
@@ -50,7 +49,7 @@ class Test {
 
     }
 
-    fun foo2(s: String) {
+    fun foo2(s: String?) {
 
     }
 

j2k/testData/fileOrElement/nullability/nullableInitializer1.kt

@@ -1,20 +1,18 @@
-// ERROR: Type mismatch: inferred type is String? but String was expected
-// ERROR: Type mismatch: inferred type is String? but String was expected
 class Test {
     fun nullableString(p: Int): String? {
         return if (p > 0) "response" else null
     }
 
-    private var nullableInitializerField: String = nullableString(3)
+    private var nullableInitializerField = nullableString(3)
     private val nullableInitializerFieldFinal = nullableString(3)
-    var nullableInitializerPublicField: String = nullableString(3)
+    var nullableInitializerPublicField = nullableString(3)
 
     fun testProperty() {
         nullableInitializerField = "aaa"
 
-        nullableInitializerField[0]
+        nullableInitializerField!![0]
         nullableInitializerFieldFinal!![0]
-        nullableInitializerPublicField[0]
+        nullableInitializerPublicField!![0]
     }
 
     fun testLocalVariable() {