[llvm] [Transforms] Speed up SSAUpdater::FindExistingPHI (PR #100281)

[William Junda Huang via llvm-commits]

huangjd wrote:

> CTMark: http://llvm-compile-time-tracker.com/compare.php?from=055e4319112282354327af9908091fdb25149e9b&to=3b2ffc540c6c5a6794d6a49d3c6e08f154dc864a&stat=instructions:u
> 
> ```
> Top 5 improvements:
>   cmake/decompress.c.ll 3546427236 3400934605 -4.10%
>   abc/decompress.c.ll 3486817506 3348637490 -3.96%
>   postgres/jsonpath_scan.ll 815735776 791156154 -3.01%
>   gromacs/gmx_hbond.cpp.ll 6368700193 6185977196 -2.87%
>   gromacs/listed_forces.cpp.ll 1362674827 1330912186 -2.33%
> Top 5 regressions:
>   lightgbm/application.cpp.ll 5131173948 5479669873 +6.79%
>   faiss/IndexIVFFlat.cpp.ll 1789637168 1903174530 +6.34%
>   meshlab/quadric_simp.cpp.ll 23870487055 24679592398 +3.39%
>   lightgbm/gradient_discretizer.cpp.ll 1339310178 1384535800 +3.38%
>   spike/regnames.ll 466569899 479275741 +2.72%
> Overall: 0.00104228%
> ```
> 
> Compile-time result looks worse on my dataset :( baseline: [2d6ff0c](https://github.com/llvm/llvm-project/commit/2d6ff0c533aa1b0fe6d91764846f636c4a4e911c)

I didn't see a significant change in compilation time on the link you provided. How do you get these data? 

https://github.com/llvm/llvm-project/pull/100281