[clang] [CodeGen] Refactor accumulateBitFields into two passes (PR #182814)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> Look, you removed some local variables and added some other local variables and a local type. It's not an objective simplification just because you personally understand your new code better. I don't think Clang should take this patch, and I am doing you the courtesy of explaining that rather than leaving you hanging. Of course, Eli may think otherwise, and if he wants to review it for you, I won't object.

Thanks for the explanation. I understand your opinion, even if I disagree with it.

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@efriedma-quic @urnathan Would you be willing to take a look? I need to modify the merging heuristics (to work around a regression from #65742 - I need to avoid non-i64 integer types for a specific use case) and the current code makes it really difficult to modify. The new code and the two-pass structure (+160 -209 lines) makes the merging logic isolated and easier to adjust.

https://github.com/llvm/llvm-project/pull/182814