[llvm] [RFC] Extend FASTCC to use up to 22 registers under APX (PR #164638)

[Shengchen Kan via llvm-commits]

KanRobert wrote:

> The caller and callee both need to have EGPR enabled for this to work. Right?

I believe so, both the caller and callee need to know where the argument is.

https://github.com/llvm/llvm-project/pull/164638