[llvm] [SmallPtrSet] Optimize Grow and inline FindBucketFor. NFC (PR #206304)

[Fangrui Song via llvm-commits]

MaskRay wrote:

https://llvm-compile-time-tracker.com/compare.php?from=49de424f45389cb757c3cc8c50daf38d024e2314&to=e3ef2d9628c2de137a06e4757ab43629f09a68d1&stat=instructions:u

Significant (≥3σ vs. measured noise): 🟢 improvement · 🔴 regression. Unmarked = within noise.

| Configuration | instructions:u | max-rss |
|---|---:|---:|
| stage1-O3 | 🟢 **-0.03%** | -0.01% |
| stage1-ReleaseThinLTO | 🟢 **-0.04%** | -0.01% |
| stage1-ReleaseLTO-g | 🟢 **-0.04%** | -0.01% |
| stage1-O0-g | -0.00% | +0.17% |
| stage1-aarch64-O3 | -0.03% | +0.13% |
| stage1-aarch64-O0-g | -0.01% | -0.06% |
| stage2-O3 | -0.06% | +0.02% |
| stage2-O0-g | +0.01% | +0.19% |

Clang build:

| Metric | Old | New | Δ |
|---|---:|---:|---:|
| instructions:u | 19895688M | 19895931M | +0.00% |
| wall-time | 335.59s | 335.45s | -0.04% |
| size-file | 137625KiB | 137640KiB | 🔴 **+0.01%** |
| size-file (stage1) | 154115KiB | 154115KiB | -0.00% |

https://github.com/llvm/llvm-project/pull/206304