[clang] [clang] Add nuw attribute to GEPs (PR #105496)

[Hari Limaye via cfe-commits]

hazzlim wrote:

> @mikaelholmen Thanks for the reproducer, this makes the issue clear. BasicAA is incorrectly returning NoAlias for the pointers due to #98608.
> 
> The issue is that the `add` gets decomposed, but we preserve the nuw flag, effectively making it an `add nuw i16 n, -1`.

Thanks for identifying this - I'm looking into a fix. Would you prefer I revert #98608 in the meantime @nikic ?

https://github.com/llvm/llvm-project/pull/105496