[llvm] [ARM] Fix CMSE S->NS calls when CONTROL_S.SFPA==0 (CVE-2024-7883) (PR #114433)

[Oliver Stannard via llvm-commits]

ostannard wrote:

The pre-commit failures are an unrelated problem on main, they went away locally after merging again.

> It might be worth adding a testcase where an function argument is undef/poison, to make sure we don't optimize out the "no-op" there?

The extra `vmov` instruction is only emitted when there are no floating-point arguments, so we don't need to worry about `s0` being poison/undef, but I'll add some tests with poison and undef integer arguments to be sure.

https://github.com/llvm/llvm-project/pull/114433