[llvm] [X86] Record the enclosed register in X86DomainReassignment::buildClosure (PR #202534)

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mbhade-amd wrote:

> test case?

This is NFC for generated code (post-isel MIR is identical; only redundant, already-discarded duplicate closures are removed), so there's no FileCheck codegen difference to assert. I can add a -stats regression test backed by a new NumClosuresBuilt statistic (takes "closures built" 12→2 on a representative function, converted unchanged) — happy to include that, or to land it as NFC without a test if you'd prefer not to add a statistic just for testing. Let me know which you'd like.

https://github.com/llvm/llvm-project/pull/202534