[llvm] [AArch64] Fix case of 0 dynamic alloc when stack probing (PR #74806)

[Oskar Wirga via llvm-commits]

oskarwirga wrote:

> How about just changing the final `str xzr, [sp]` to `ldr xzr, [sp]`? Thoughts?

I believe we discussed something like this in an earlier revision of your PR, I like it better. Do we need to worry about optimizations removing it? Should we change other probes to use it? 

https://github.com/llvm/llvm-project/pull/74806