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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

> Right, one describes the struct types for aliasing, the other seems to be used for padding:

We should probably try to unify this at some point... but I guess we can put this aside for now.

> > If we fail to extract the field from the tbaa.struct, should we drop the tbaa.struct metadata?
> 
> Intuitively, I think it could make sense. As far as tbaa.struct is concerned, part of the users in the middle-end seem to be only transferring the metadata into the new memcpy / memmove. Not sure on this though.

I'd like to work this out now, while we're looking at when tbaa.struct needs to be nulled-out... what are the required invariants here?

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