Adopt let_else across the compiler

This performs a substitution of code following the pattern:

let <id> = if let <pat> = ... { identity } else { ... : ! };

To simplify it to:

let <pat> = ... { identity } else { ... : ! };

By adopting the let_else feature.

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -716,9 +716,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             cause.clone(),
         );
 
-        let data = if let ty::Dynamic(ref data, ..) = normalized_ty.kind() {
-            data
-        } else {
+        let ty::Dynamic(data, ..) = normalized_ty.kind() else {
             return None;
         };