Fix apply in internal if for pull_assignment_up

Example
---
```rust
fn foo() {
    let mut a = 1;

    if true {
        a = 2;
    } else if true {
        $0a = 3;
    } else {
        a = 4;
    }
}
```

**Before this PR**:

```rust
fn foo() {
    let mut a = 1;

    if true {
        a = 2;
    } else a = if true {
        3
    } else {
        4
    };
}
```

**After this PR**:

```rust
fn foo() {
    let mut a = 1;

    a = if true {
        2
    } else if true {
        3
    } else {
        4
    };
}
```

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

@@ -53,6 +53,10 @@ pub(crate) fn pull_assignment_up(acc: &mut Assists, ctx: &AssistContext<'_>) ->
     };
 
     let tgt: ast::Expr = if let Some(if_expr) = ctx.find_node_at_offset::<ast::IfExpr>() {
+        let if_expr = std::iter::successors(Some(if_expr), |it| {
+            it.syntax().parent().and_then(ast::IfExpr::cast)
+        })
+        .last()?;
         collector.collect_if(&if_expr)?;
         if_expr.into()
     } else if let Some(match_expr) = ctx.find_node_at_offset::<ast::MatchExpr>() {
@@ -237,6 +241,37 @@ fn foo() {
         );
     }
 
+    #[test]
+    fn test_pull_assignment_up_inner_if() {
+        check_assist(
+            pull_assignment_up,
+            r#"
+fn foo() {
+    let mut a = 1;
+
+    if true {
+        a = 2;
+    } else if true {
+        $0a = 3;
+    } else {
+        a = 4;
+    }
+}"#,
+            r#"
+fn foo() {
+    let mut a = 1;
+
+    a = if true {
+        2
+    } else if true {
+        3
+    } else {
+        4
+    };
+}"#,
+        );
+    }
+
     #[test]
     fn test_pull_assignment_up_match() {
         check_assist(