[llvm] [GenericDomTreeConstruction] Speed up Semi-NCA construction (PR #207474)

[Fangrui Song via llvm-commits]

MaskRay wrote:

https://llvm-compile-time-tracker.com/compare.php?from=bf68b4165c419f1893d0668bb05630e33c2efe8c&to=8bbd8136542ca479c042b4f0f1f22d7cd2d2113b&stat=instructions:u

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

| Configuration | instructions:u | max-rss |
|---|---:|---:|
| stage1-O3 | 🟢 **-0.31%** | -0.03% |
| stage1-ReleaseThinLTO | 🟢 **-0.32%** | -0.01% |
| stage1-ReleaseLTO-g | 🟢 **-0.28%** | +0.06% |
| stage1-O0-g | +0.00% | -0.03% |
| stage1-aarch64-O3 | 🟢 **-0.30%** | +0.15% |
| stage1-aarch64-O0-g | -0.02% | -0.05% |
| stage2-O3 | 🟢 **-0.21%** | +0.01% |
| stage2-O0-g | +0.00% | -0.02% |

Clang build:

| Metric | Old | New | Δ |
|---|---:|---:|---:|
| instructions:u | 19983736M | 19933337M | 🟢 **-0.25%** |
| wall-time | 336.40s | 334.64s | -0.52% |
| size-file | 137832KiB | 137834KiB | 🔴 **+0.00%** |
| size-file (stage1) | 154414KiB | 154414KiB | -0.00% |


https://github.com/llvm/llvm-project/pull/207474