[llvm] [Support][ADT] Minor cleanups after #101706 (PR #102180)

[Jakub Kuderski via llvm-commits]

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@@ -262,7 +262,9 @@ class DominatorTreeBase {
       SmallVector<std::unique_ptr<DomTreeNodeBase<NodeT>>>;
   DomTreeNodeStorageTy DomTreeNodes;
   // For graphs where blocks don't have numbers, create a numbering here.
-  DenseMap<const NodeT *, unsigned> NodeNumberMap;
+  std::conditional_t<!GraphHasNodeNumbers<NodeT *>,
+                     DenseMap<const NodeT *, unsigned>, std::tuple<>>
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kuhar wrote:

nit: consider adding todo to switch to an empty struct with `no_unique_address ` once we update to C++20.

https://github.com/llvm/llvm-project/pull/102180