Introduce Variable: Properly convert function body to block when needed

#KT-5353

idea/testData/refactoring/introduceVariable/FunctionAddBlock.kt.after

@@ -1,3 +1,4 @@
 fun x(): Int {
     val i = 1
+    return i
 }

idea/testData/refactoring/introduceVariable/FunctionAddBlockInner.kt

@@ -1 +1,6 @@
-fun x(): Int = println(<selection>1</selection>)
+fun foo(a: Int) =
+        if (a > 1) {
+            (<selection>a + 1</selection>) * (a - 1)
+        } else {
+            a * (a + 1)
+        }

idea/testData/refactoring/introduceVariable/FunctionAddBlockInner.kt.after

@@ -1,4 +1,8 @@
-fun x(): Int {
-    val i = 1
-    println(i)
+fun foo(a: Int): Int {
+    val i = a + 1
+    return if (a > 1) {
+        i * (a - 1)
+    } else {
+        a * i
+    }
 }