[llvm] [GlobalOpt] Add TTI interface useFastCCForInternalCall for FASTCC (PR #164768)

[Nikita Popov via llvm-commits]

nikic wrote:

Hm, I'm not sure I like this. Would it make sense to potentially have multiple fastcc calling conventions (e.g. fastcc_egpr) and have the hook select the best one? For this specific case (where fastcc doesn't do anything on the baseline target) this wouldn't make a difference, but this would also accommodate other cases in the future.

https://github.com/llvm/llvm-project/pull/164768