[clang] [llvm] [RegAlloc] Refactor CSR cost interface and enable CSR cost for X86 (PR #202007)

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weiguozhi wrote:

> > Is this tested with FDO or plain mode?
> 
> plain mode

Could you also test it with PGO?
With profiling information, I expect LLVM computes more precise CSRCost and spillcost, so we can get better result.

https://github.com/llvm/llvm-project/pull/202007