[llvm] [FuncSpec] Update function specialization to handle phi-chains (PR #71442)

[Alexandros Lamprineas via llvm-commits]

https://github.com/labrinea commented:

I've added some inline comments about the debug output, otherwise looks fine. I would like @momchil-velikov to have a say since I have co-authored this patch. I checked the compile time impact on the llvm-test-suite, which didn't raise any concerns (same amount of specializations happening, 2 specializations in ClamAV for the O3 pipeline, 2 specializations in SPASS for the LTO pipeline; geomean was +0.028% for the O3 pipeline and +0.012% for the full LTO pipeline on x86)

https://github.com/llvm/llvm-project/pull/71442