[clang] [llvm] [X86] Extend kCFI with a 3-bit arity indicator (PR #121070)

[Phoebe Wang via llvm-commits]

phoebewang wrote:

And you cannot use ESI when it's used to pass arguments. Suggested registers:
| Arity Indicator | Description | Encoding in reg field |
| --------------- | --------------- | --------------- |
| 0 | 0 parameters | EDI |
| 1 | 1 parameter in RDI | ESI |
| 2 | 2 parameters in RDI and RSI | EDX |
| 3 | 3 parameters in RDI, RSI, and RDX | ECX |
| 4 | 4 parameters in RDI, RSI, RDX, and RCX | R8D |
| 5 | 5 parameters in RDI, RSI, RDX, RCX, and R8 | R9D |
| 6 | 6 parameters in RDI, RSI, RDX, RCX, R8, and R9 | R10D |
| 7 | At least one parameter may be passed on the stack | EAX |


https://github.com/llvm/llvm-project/pull/121070