[ADT] Fix a bug in EquivalenceClasses::erase#161121

kazutakahirata merged 3 commits intollvm:mainllvm/llvm-project:mainfrom

kazutakahirata:cleanup_20250928_ADT_EquivalenceClasses_nullptrkazutakahirata/llvm-project:cleanup_20250928_ADT_EquivalenceClasses_nullptrCopy head branch name to clipboard