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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

Why do we have both tbaa and tbaa.struct on the same instruction in the first place?  That doesn't really seem useful for describing the aliasing; I'd think one or the other would be enough.  I guess the tbaa.struct allows describing the gaps?

If we do expect to have both tbaa and tbaa.struct available, in general, is there any point to trying to extract the tbaa info from the tbaa.struct?

If we fail to extract the field from the tbaa.struct, should we drop the tbaa.struct metadata?

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