Avoid checking the edition as much as possible

Inside #123865, we are adding support for the new semantics
for expr2024, but we have noted a performance issue.

We realized there is a redundant check for each
token regarding an edition. This commit moves the edition
check to the end, avoiding some extra checks that
can slow down compilation time.

Link: #123865
Co-Developed-by: @eholk
Signed-off-by: Vincenzo Palazzo <[email protected]>

compiler/rustc_parse/src/parser/nonterminal.rs

@@ -47,7 +47,7 @@ impl<'a> Parser<'a> {
                 token.can_begin_expr()
                 // This exception is here for backwards compatibility.
                 && !token.is_keyword(kw::Let)
-                && (token.span.edition().at_least_rust_2024() || !token.is_keyword(kw::Const))
+                && (!token.is_keyword(kw::Const) || token.span.edition().at_least_rust_2024())
             }
             NonterminalKind::Ty => token.can_begin_type(),
             NonterminalKind::Ident => get_macro_ident(token).is_some(),