[llvm] [AArch64] Reland merge index address with large offset into base address (PR #79951)

[via llvm-commits]

vfdff wrote:

> Thanks. Do you know if the issue from #76202 is still present, or what it might have been caused by? It sounded like a mis-compile.

Oh, yes. I should also wait to figure out this issue, but now I don't have any idea. 
I haven't had a smooth reproduction of the problem, and will take some more trying.

https://github.com/llvm/llvm-project/pull/79951