[llvm] [LoopInfo] Reuse DomTree's own DFS numbering in analyze() (PR #207650)

[Fangrui Song via llvm-commits]

MaskRay wrote:

% ruby ~/bin/ct_table.rb 'https://llvm-compile-time-tracker.com/compare.php?from=deecf8dce7dd6df5ce82bf273f7bfb75a783160a&to=115ba7183436ca4d591fd86e1309b71a8d4b0a20&stat=instructions:u'

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

| Configuration | instructions:u | max-rss |
|---|---:|---:|
| stage1-O3 | 🟢 **-0.06%** | +0.01% |
| stage1-ReleaseThinLTO | 🟢 **-0.06%** | +0.02% |
| stage1-ReleaseLTO-g | 🟢 **-0.06%** | +0.01% |
| stage1-O0-g | +0.01% | +0.02% |
| stage1-aarch64-O3 | 🟢 **-0.05%** | +0.02% |
| stage1-aarch64-O0-g | -0.00% | -0.06% |
| stage2-O3 | -0.04% | +0.09% |
| stage2-O0-g | +0.01% | -0.05% |

Clang build:

| Metric | Old | New | Δ |
|---|---:|---:|---:|
| instructions:u | 19900884M | 19896311M | -0.02% |
| wall-time | 334.82s | 335.71s | +0.27% |
| size-file | 137735KiB | 137733KiB | 🟢 **-0.00%** |
| size-file (stage1) | 154262KiB | 154262KiB | +0.00% |

https://github.com/llvm/llvm-project/pull/207650