[llvm] [IR] Reduce memory usage in metadata use tracking (NFC) (PR #221866)

[Fangrui Song via llvm-commits]

MaskRay wrote:

dropRef() in the latest revision is still O(N).

I've put up #227186 as a draft for reference, not to compete with this PR ("Squash and merge" uses the submitter's account as the author where every other author is Co-authored-by). I'd suggest adopting its `dropRef` design here; please take whatever is useful (`replaceAllUsesWith`, reverse is in findRef).

It also makes the std::reverse in replaceAllUsesWith unnecessary, since a drop from the front costs the same as a drop from the back. The index is freed at 16 entries and rebuilt past 32, so it can't thrash.

 On a function with 50,000 assignments to one local (assignment tracking drains the alloca's dbg_assign uses oldest first), same base:

```
 main (DenseMap)     9.1 s
 #227186             9.3 s
 plain SmallVector  10.5 s
 this PR (v3)       21.2 s
```
 
#227186 also adds a unit test for lists that cross the index threshold. Only asserts catch a stale index entry, and they only trip if the changed reference is looked up again right away, because a later swap in dropRef repairs the entry. I checked it against five index-bookkeeping bugs (a missing index erase or store in addRef, dropRef or moveRef), and it catches all five; the existing tests catch none. Feel free to take it as is.


https://github.com/llvm/llvm-project/pull/221866