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

[Craig Topper via llvm-commits]

topperc wrote:

> I think the proposal here is different with preserve_none and other dedicated calling conventions. This patch only applies to the cases of internal function optimizations: https://godbolt.org/z/3xT94Wvjv. So we don't have a subtarget different issue.
> 
> In the example of https://godbolt.org/z/fq99T6h63, I showed Clang already warns for the use of fastcc in C side. We may consider to relex it to allowing used with APX only. So we don't have a subtarget different issue even we want to expend it.

`internal` doesn't guarantee the subtarget is the same. Can't target attribute or LTO create a mismatch in subtarget?

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