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

[Phoebe Wang via llvm-commits]

phoebewang 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.

I don't think multiple fastcc calling conventions is useful, based on the fact that we cannot bind calling convention with a single feature. Assume we have multiple features in the future, the solution is still to disable fastcc instead of allowing multiple calling conventions coexist.

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