[llvm] [DenseMap] Share rehash and grow for relocatable bucket types. NFC (PR #225018)

[Fangrui Song via llvm-commits]

MaskRay wrote:

% ct_table.rb 'https://llvm-compile-time-tracker.com/compare.php?from=63225d3616be90ab15b97919ac12ae0ab052ac82&to=1d25beedb8610f37b20ae71d95f209455fb97bae&stat=instructions:u'
CTMark geomean `instructions:u`: 🟢 3 improved (best -0.06%) · 🔴 1 regressed (worst +0.05%) · 4 within noise.

| Config                | instructions:u |   σ |       max-rss |
| :-------------------- | -------------: | --: | ------------: |
| stage1-O3             |  🟢 **-0.04%** | 3.6 | 🟢 **-0.79%** |
| stage1-ReleaseThinLTO |  🟢 **-0.05%** | 5.0 | 🟢 **-0.85%** |
| stage1-ReleaseLTO-g   |         +0.00% | 0.1 | 🟢 **-0.63%** |
| stage1-O0-g           |  🔴 **+0.05%** | 3.0 | 🟢 **-0.81%** |
| stage1-aarch64-O3     |  🟢 **-0.06%** | 5.3 | 🟢 **-1.02%** |
| stage1-aarch64-O0-g   |         +0.03% | 1.6 | 🟢 **-0.97%** |
| stage2-O3             |         -0.04% | 2.2 | 🟢 **-0.54%** |
| stage2-O0-g           |         +0.02% | 0.7 |        -0.24% |

Building clang itself (stage2, ThinLTO):

| Metric             |       Old |       New |             Δ |    σ |
| :----------------- | --------: | --------: | ------------: | ---: |
| instructions:u     | 18942497M | 18780198M | 🟢 **-0.86%** |   41 |
| wall-time          |   324.47s |   319.49s |        -1.53% |  2.0 |
| size-file          | 136859KiB | 135983KiB | 🟢 **-0.64%** |    ∞ |
| size-file (stage1) | 155494KiB | 154176KiB | 🟢 **-0.85%** | 1318 |

https://github.com/llvm/llvm-project/pull/225018