[AArch64] Fix case of 0 dynamic alloc when stack probing (#74877)

 I accidentally closed
https://github.com/llvm/llvm-project/pull/74806

If the dynamic allocation size is 0, then we will still probe the
current sp value despite not decrementing sp! This results in
overwriting stack data, in my case the stack canary.

The fix here is just to load the value of [sp] into xzr which is
essentially a no-op but still performs a read/probe of the new page.