Adopt let_else in even more places

compiler/rustc_index/src/bit_set.rs

@@ -852,11 +852,7 @@ impl<T: Idx> HybridBitSet<T> {
             Bound::Excluded(end) => end.index(),
             Bound::Unbounded => self.domain_size() - 1,
         };
-        let len = if let Some(l) = end.checked_sub(start) {
-            l
-        } else {
-            return;
-        };
+        let Some(len) = end.checked_sub(start) else { return };
         match self {
             HybridBitSet::Sparse(sparse) if sparse.len() + len < SPARSE_MAX => {
                 // The set is sparse and has space for `elems`.