refactor(transformer): RegExp transform unbox early (#5504)

Unbox the `RegExpLiteral` early to avoid all lookups thereafter going through the indirection of a `Box`.

crates/oxc_transformer/src/regexp/mod.rs

@@ -113,9 +113,10 @@ impl<'a> Traverse<'a> for RegExp<'a> {
         expr: &mut Expression<'a>,
         ctx: &mut oxc_traverse::TraverseCtx<'a>,
     ) {
-        let Expression::RegExpLiteral(ref mut regexp) = expr else {
+        let Expression::RegExpLiteral(regexp) = expr else {
             return;
         };
+        let regexp = regexp.as_mut();
 
         let flags = regexp.regex.flags;
         let has_unsupported_flags = flags.intersects(self.unsupported_flags);