perf(linter): reduce branches-sharing-code scratch allocations (#28483)

Get rid of 1 Vec that was just used for scratch, and use smallvec for
the other Vec.

On local fixtures:

Allocations: 24,658 → 346 (−98.6%)
Allocated bytes: 797,698 → 18,466 (−97.7%)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

crates/oxc_linter/src/rules/oxc/branches_sharing_code.rs

@@ -1,7 +1,7 @@
 // Inspired by https://github.com/rust-lang/rust-clippy/blob/95f5a00d8628fba223c802251af3c42547927c5a/clippy_lints/src/ifs/branches_sharing_code.rs
 use oxc_ast::{
     AstKind,
-    ast::{Expression, IdentifierReference, IfStatement, Statement},
+    ast::{IdentifierReference, IfStatement, Statement},
 };
 use oxc_ast_visit::Visit;
 use oxc_diagnostics::OxcDiagnostic;
@@ -10,6 +10,7 @@ use oxc_macros::declare_oxc_lint;
 use oxc_semantic::{ReferenceId, SymbolId};
 use oxc_span::{ContentEq, GetSpan, Span};
 use rustc_hash::FxHashSet;
+use smallvec::SmallVec;
 
 use crate::{AstNode, ast_util::get_preceding_indent_str, context::LintContext, rule::Rule};
 
@@ -111,11 +112,9 @@ impl Rule for BranchesSharingCode {
             return;
         }
 
-        let (conditions, bodies) = extract_if_sequence(if_stmt);
-
-        if bodies.len() < 2 || bodies.len() == conditions.len() {
+        let Some(bodies) = extract_if_sequence(if_stmt) else {
             return;
-        }
+        };
 
         let (start_eq, end_eq) = scan_blocks_for_eq(&bodies);
 
@@ -341,30 +340,23 @@ fn scan_blocks_for_eq<'a>(bodies: &[&'a Statement<'a>]) -> (Option<usize>, Optio
 
 fn extract_if_sequence<'a>(
     if_stmt: &'a IfStatement<'a>,
-) -> (Vec<&'a Expression<'a>>, Vec<&'a Statement<'a>>) {
-    let mut conditions = Vec::new();
-    let mut bodies = Vec::new();
+) -> Option<SmallVec<[&'a Statement<'a>; 2]>> {
+    let mut bodies = SmallVec::new();
 
-    let mut current_if = Some(if_stmt);
-    while let Some(if_node) = current_if {
-        conditions.push(&if_node.test);
-        bodies.push(&if_node.consequent);
+    let mut current_if = if_stmt;
+    loop {
+        bodies.push(&current_if.consequent);
 
-        match &if_node.alternate {
-            Some(Statement::IfStatement(else_if)) => {
-                current_if = Some(else_if);
+        match current_if.alternate.as_ref()? {
+            Statement::IfStatement(else_if) => {
+                current_if = else_if;
             }
-            Some(else_stmt) => {
+            else_stmt => {
                 bodies.push(else_stmt);
-                current_if = None;
-            }
-            None => {
-                current_if = None;
+                return Some(bodies);
             }
         }
     }
-
-    (conditions, bodies)
 }
 
 #[test]