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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

I sort of understand the logic of "we need a calling convention, fastcc isn't useful on x86-64, let's repurpose it".  But there's a significant benefit to giving a calling convention with unusual semantics (like, it only works on targets that have APX enabled) its own name; it's less confusing, and it cleanly allows future extensions for all targets.  And introducing a new calling convention really isn't that much more work.

It isn't likely the set of integer registers on x86 will change again, but floating-point/vector registers have been getting constant updates.

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