[llvm] [TBAA] Do not rewrite TBAA tag in `adjustForAccess` if already exists (PR #96483)

[Antonio Frighetto via llvm-commits]

antoniofrighetto wrote:

Kind ping. @nikic AFAICT I think this should basically reinstate behaviour prior to a87fa7f0caa1b8bf328524e0aec80a10ed4af7f9, modulo for taking into account scope/noalias too (as long as it was previously correct).

https://github.com/llvm/llvm-project/pull/96483