Fixed codegen for single-branch if

#KT-2597 Fixed
 #KT-2598 Fixed

compiler/testData/codegen/regressions/kt2597.kt

@@ -0,0 +1,10 @@
+fun box(): String {
+    var i = 0
+    {
+        if (1 == 1) {
+            i++
+        } else {
+        }
+    }()
+    return "OK"
+}

compiler/testData/codegen/regressions/kt2598.kt

@@ -0,0 +1,9 @@
+fun foo(condition: Boolean): String {
+    val u = if (condition) {
+        "OK"
+    } else {
+    }
+    return u.toString()
+}
+
+fun box() = foo(true)