Fix applicable on let-else for convert_to_guarded_return

Example
---
```rust
fn foo() -> bool {
    let$0 Some(x) = Some(2) else { return false };
}
```

**Before this PR**:

```rust
fn foo() -> bool {
    let Some(Some(x)) = Some(2) else { return };
}
```

**After this PR**:

Assist not applicable

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

@@ -182,7 +182,7 @@ fn let_stmt_to_guarded_return(
     let cursor_in_range =
         let_token_range.cover(let_pattern_range).contains_range(ctx.selection_trimmed());
 
-    if !cursor_in_range {
+    if !cursor_in_range || let_stmt.let_else().is_some() {
         return None;
     }
 
@@ -911,6 +911,19 @@ fn main() {
         );
     }
 
+    #[test]
+    fn ignore_let_else_branch() {
+        check_assist_not_applicable(
+            convert_to_guarded_return,
+            r#"
+//- minicore: option
+fn main() {
+    let$0 Some(x) = Some(2) else { return };
+}
+"#,
+        );
+    }
+
     #[test]
     fn ignore_statements_after_if() {
         check_assist_not_applicable(