Simplify and optimize SlotIndex::from_index

Break out bucket 0 (containing `idx < 4096`) as an early return, which
simplifies the remainder of the function, and allows optimizing the
`checked_ilog2` since it can no longer return `None`.

This reduces the runtime of `vec_cache::tests::slot_index_exhaustive`
(which calls `SlotIndex::from_index` for every `u32`, twice) from ~15.5s
to ~13.3s.

compiler/rustc_data_structures/src/vec_cache.rs

@@ -68,22 +68,13 @@ impl SlotIndex {
     // slots (see `slot_index_exhaustive` in tests).
     #[inline]
     const fn from_index(idx: u32) -> Self {
-        let mut bucket = match idx.checked_ilog2() {
-            Some(x) => x as usize,
-            None => 0,
-        };
-        let entries;
-        let running_sum;
-        if bucket <= 11 {
-            entries = 1 << 12;
-            running_sum = 0;
-            bucket = 0;
-        } else {
-            entries = 1 << bucket;
-            running_sum = entries;
-            bucket = bucket - 11;
+        if idx < 4096 {
+            return SlotIndex { bucket_idx: 0, entries: 4096, index_in_bucket: idx as usize };
         }
-        SlotIndex { bucket_idx: bucket, entries, index_in_bucket: idx as usize - running_sum }
+        // SAFETY: We already ruled out idx 0, so `checked_ilog2` can't return `None`.
+        let bucket = unsafe { idx.checked_ilog2().unwrap_unchecked() as usize };
+        let entries = 1 << bucket;
+        SlotIndex { bucket_idx: bucket - 11, entries, index_in_bucket: idx as usize - entries }
     }
 
     // SAFETY: Buckets must be managed solely by functions here (i.e., get/put on SlotIndex) and