Replace 'if' with 'when': don't swallow comments if there is no synthetic else branch

#KT-34640 Fixed

idea/src/org/jetbrains/kotlin/idea/intentions/branchedTransformations/intentions/IfToWhenIntention.kt

@@ -168,7 +168,11 @@ class IfToWhenIntention : SelfTargetingRangeIntention<KtIfExpression>(KtIfExpres
                 val currentElseBranch = currentIfExpression.`else`
                 if (currentElseBranch == null) {
                     // Try to build synthetic if / else according to KT-10750
-                    val syntheticElseBranch = if (canPassThrough) break else buildNextBranch(baseIfExpressionForSyntheticBranch) ?: break
+                    val syntheticElseBranch = if (canPassThrough) null else buildNextBranch(baseIfExpressionForSyntheticBranch)
+                    if (syntheticElseBranch == null) {
+                        applyFullCommentSaver = false
+                        break
+                    }
                     toDelete.addAll(baseIfExpressionForSyntheticBranch.siblingsUpTo(syntheticElseBranch))
                     if (syntheticElseBranch is KtIfExpression) {
                         baseIfExpressionForSyntheticBranch = syntheticElseBranch

idea/testData/intentions/branched/ifWhen/ifToWhen/doNotSwallowComment.kt

@@ -0,0 +1,12 @@
+fun test(foo: String) {
+    <caret>if (foo == "1") {
+        println("1")
+    }
+
+    if (foo == "a") {
+        // some comments for "a"
+        println("a");
+    }
+}
+
+fun println(s: String) {}

idea/testData/intentions/branched/ifWhen/ifToWhen/doNotSwallowComment.kt.after

@@ -0,0 +1,14 @@
+fun test(foo: String) {
+    when (foo) {
+        "1" -> {
+            println("1")
+        }
+    }
+
+    if (foo == "a") {
+        // some comments for "a"
+        println("a");
+    }
+}
+
+fun println(s: String) {}

idea/tests/org/jetbrains/kotlin/idea/intentions/IntentionTestGenerated.java

@@ -2327,6 +2327,11 @@ public class IntentionTestGenerated extends AbstractIntentionTest {
                     runTest("idea/testData/intentions/branched/ifWhen/ifToWhen/comment.kt");
                 }
 
+                @TestMetadata("doNotSwallowComment.kt")
+                public void testDoNotSwallowComment() throws Exception {
+                    runTest("idea/testData/intentions/branched/ifWhen/ifToWhen/doNotSwallowComment.kt");
+                }
+
                 @TestMetadata("ifElseSwallowComments.kt")
                 public void testIfElseSwallowComments() throws Exception {
                     runTest("idea/testData/intentions/branched/ifWhen/ifToWhen/ifElseSwallowComments.kt");