feat: offer 'add_braces' on bin-expr assignment

Example
---
```rust
fn foo() {
    let x;
    x =$0 n + 100;
}
```

**Before this PR**

Assist not applicable

**After this PR**

```rust
fn foo() {
    let x;
    x = {
        n + 100
    };
}
```

src/tools/rust-analyzer/crates/ide-assists/src/handlers/add_braces.rs

@@ -84,6 +84,7 @@ fn get_replacement_node(ctx: &AssistContext<'_>) -> Option<(ParentType, ast::Exp
             match parent {
                 ast::LetStmt(it) => it.initializer()?,
                 ast::LetExpr(it) => it.expr()?,
+                ast::BinExpr(it) => it.rhs()?,
                 ast::Static(it) => it.body()?,
                 ast::Const(it) => it.body()?,
                 _ => return None,
@@ -192,6 +193,22 @@ fn foo() {
         n + 100
     };
 }
+"#,
+        );
+
+        check_assist(
+            add_braces,
+            r#"
+fn foo() {
+    if let x =$0 n + 100 {}
+}
+"#,
+            r#"
+fn foo() {
+    if let x = {
+        n + 100
+    } {}
+}
 "#,
         );
     }