[llvm] [SelectionDAG] Unique VT lists in a DenseSet instead of a FoldingSet (PR #219364)

[Fangrui Song via llvm-commits]

MaskRay wrote:

https://llvm-compile-time-tracker.com/compare.php?from=1967eae7a5c4865ff3cf425b111ee64a1d54befd&to=7f5b304f21fc1aa61a403182db6443b190379672&stat=instructions:u

```
Commit	stage1-O3	stage1-ReleaseThinLTO	stage1-ReleaseLTO-g	stage1-O0-g	stage1-aarch64-O3	stage1-aarch64-O0-g	stage2-O3	stage2-O0-g	stage2-clang
55237M (-0.11%)	69401M (-0.08%)	79412M (-0.08%)	16608M (-0.01%)	60280M (-0.05%)	18906M (+0.00%)	48474M (-0.06%)	14735M (-0.04%)	19294830M (-0.04%)
```

https://github.com/llvm/llvm-project/pull/219364