[llvm] [ADT] Fix a bug in EquivalenceClasses::erase (PR #161121)

[Kazu Hirata via llvm-commits]

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@@ -258,7 +258,8 @@ template <class ElemTy> class EquivalenceClasses {
         // If the current element is the last element(not leader), set the
         // successor of the current element's predecessor to null, and set
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kazutakahirata wrote:

I've inserted `while preserving the leader bit` there.  How does this sound?

https://github.com/llvm/llvm-project/pull/161121