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

[Thurston Dang via llvm-commits]

thurstond wrote:

P.S. the error on the buildbots was consistently
```
==1541260==ERROR: UndefinedBehaviorSanitizer: SEGV on unknown address 0x220022002324 (pc 0xb07874395294 bp 0xffffede79610 sp 0xffffede795b0 T1541260)
```
e.g., https://lab.llvm.org/buildbot/#/builders/85/builds/1102, https://lab.llvm.org/buildbot/#/builders/85/builds/1103, ..., https://lab.llvm.org/buildbot/#/builders/85/builds/1125

Address space layout randomization is enabled on the bots, so it's exceedingly unlikely to get the same memory address by chance. This suggests that, somehow, the pointer is being overwritten by some data.


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