[llvm] [ARM] Add a method to clear registers (PR #69659)

[Nick Desaulniers via llvm-commits]

https://github.com/nickdesaulniers commented:

If your intent is to next call this override from ARMFrameLowering, I think it would be best to include that change in this PR.  As is, the lack of unit tests is ...

https://github.com/llvm/llvm-project/pull/69659