[llvm] Reapply "[ARM][NFC] Use addLiveIns method instead of manually adding live ins (#87560)" (PR #96448)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

I'm not familiar with this code, but given the test diffs, this is clearly not NFC, right?

https://github.com/llvm/llvm-project/pull/96448