[llvm] [AArch64] Only clear kill flags if necessary when merging str (PR #69680)

[David Green via llvm-commits]

davemgreen wrote:

I think this sounds OK. I'm a little surprised that it can't recalc the info if needed.
Is it worth adding a test for the specific case you had - "which results in suboptimal register allocation"? Or do you think the existing tests that change enough to show the issue?

https://github.com/llvm/llvm-project/pull/69680