[llvm] EquivalenceClasses: Make ECValue public. NFC (PR #139689)

[via llvm-commits]

ruiling wrote:

> Can you share why this is needed? `ECValue` is intentionally not exposed publicly as it is an internal implementation detail

See the discussion at https://github.com/llvm/llvm-project/pull/137784#discussion_r2066220486. Using explicit type `EquivalenceClasses<SUnit *>::ECValue *` is clearer and it depends on making the inner class public.

https://github.com/llvm/llvm-project/pull/139689