[llvm] [NFC][equivalenceClass] Refactor coding style in EquivalenceClasses.h. (PR #135467)

[donald chen via llvm-commits]

cxy-1993 wrote:

This PR used clang-format to format the code style and also replaced C-style casts with C++ style casts. 

https://github.com/llvm/llvm-project/pull/135467