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

[Kazu Hirata via llvm-commits]

kazutakahirata wrote:

I'm reposting the benchmark part of the commit message here as it did not render correctly in the OP:

```
                       Base     Test
File                Max RSS  Max RSS    Delta           Wall Time
-----------------------------------------------------------------
SLPVectorizer.ii    1300 MB  1159 MB  -10.85%    43.76s -> 43.39s
SemaOpenMP.ii       1281 MB  1112 MB  -13.16%    34.29s -> 33.88s
X86ISelLowering.ii   831 MB   792 MB   -4.65%    26.24s -> 26.10s
-----------------------------------------------------------------
Total               3412 MB  3064 MB  -10.21%  104.29s -> 103.37s
Geometric Mean                         -9.62%              -0.86%

- Compilation flags: -O3 -g -fdebug-info-for-profiling -flto=thin
- Max RSS: average of 3 runs after 1 warmup run.
- Wall Time: average of 10 runs after 1 warmup run.
```


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