Replace if let with guard let (#1466)

Summary:
## Changes in this pull request

### Checklist

- [x] All tests pass. Demo project builds and runs.
- [x] I added tests, an experiment, or detailed why my change isn't tested.
- [x] I added an entry to the `CHANGELOG.md` for any breaking changes, enhancements, or bug fixes.
- [x] I have reviewed the [contributing guide](https://github.com/Instagram/IGListKit/blob/master/.github/CONTRIBUTING.md)

Pull Request resolved: https://github.com/Instagram/IGListKit/pull/1466

Reviewed By: bdotdub

Differential Revision: D24235336

Pulled By: lorixx

fbshipit-source-id: bfc5fab26ae28ff9306d1b889b82b49c6ae2a636

Source/IGListSwiftKit/ListIdentifiable.swift

@@ -33,11 +33,10 @@ public extension ListIdentifiable {
 
     // TODO(natesm): Should this be a public API? It is for now.
     init?(diffable: Any) {
-        if let value = (diffable as? ListDiffableValueBox<Self>)?.value {
-            self = value
-        } else {
+        guard let value = (diffable as? ListDiffableValueBox<Self>)?.value else {
             return nil
         }
+        self = value
     }
 }
 
@@ -63,10 +62,9 @@ private final class ListDiffableValueBox<Value: ListIdentifiable>: NSObject, Lis
     }
 
     func isEqual(toDiffableObject object: ListDiffable?) -> Bool {
-        if let other = object as? ListDiffableValueBox<Value> {
-            return value == other.value
-        } else {
+        guard let other = object as? ListDiffableValueBox<Value> else {
             return false
         }
+        return value == other.value
     }
 }

Source/IGListSwiftKit/ListValueSectionController.swift

@@ -23,12 +23,11 @@ open class ListValueSectionController<Value: ListIdentifiable>: ListSectionContr
     /// Subclasses of `ListValueSectionController` may not override `didUpdate(to:)` for objects.
     /// Instead, they must use the overload for generic values.
     public final override func didUpdate(to object: Any) {
-        if let value = Value(diffable: object) {
-            self.value = value
-            didUpdate(to: value)
-        } else {
+        guard let value = Value(diffable: object) else {
             fatalError("Expected object for value section controller to be a boxed \(Value.self), but it was \(object)")
         }
+        self.value = value
+        didUpdate(to: value)
     }
 
     /// Updates the section controller to a new value.