[llvm] [EquivalenceClasses] Introduce erase member function (PR #134660)

[Jakub Kuderski via llvm-commits]

================
@@ -220,6 +221,57 @@ template <class ElemTy> class EquivalenceClasses {
     return *ECV;
   }
 
+  /// erase - Erase a value from the union/find set, return "true" if erase
+  /// succeeded.
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kuhar wrote:

When can we get `false`?

```suggestion
  /// succeeded, or "false" when the value was not found.
```
?

https://github.com/llvm/llvm-project/pull/134660