Reverts llvm/llvm-project#119225 due to the lack of sanitizer support,
large potential of breaking code containing latent UB, non-trivial
localization and investigation, and what seems to be a bad interaction
with msan (a test is in the works).

Related discussions:
https://github.com/llvm/llvm-project/pull/119225#issuecomment-2551904822
https://github.com/llvm/llvm-project/pull/118472#issuecomment-2549986255

When we have a gep inbounds from the base of an object (e.g. alloca or
global), we know that the index cannot be negative, as this would go out
of bounds. As such, we can infer nuw as well.

The implementation is a bit stricter than necessary, we could also
accept one unknown index followed by known-non-negative indices.

Proof: https://alive2.llvm.org/ce/z/Hp7-6w (Note that alive2 currently
incorrectly doesn't require the inbounds for the alloca case, see
https://github.com/AliveToolkit/alive2/issues/1138).

This gives an improvement of 0.6%:
https://llvm-compile-time-tracker.com/compare.php?from=7d35fe6d08e2b9b786e1c8454cd2391463832167&to=0456c8e8a42be06b62ad4c3e3cf34b21f2633d1e&stat=instructions:u

Differential Revision: https://reviews.llvm.org/D158600

The unsized-pointee-crash.ll and zero-sized-pointee-crash.ll tests
have been removed, because these issues are not relevant for opaque
pointers.

IR is now always parsed in opaque pointer mode, unless
-opaque-pointers=0 is explicitly given. There is no automatic
detection of typed pointers anymore.

The -opaque-pointers=0 option is added to any remaining IR tests
that haven't been migrated yet.

Differential Revision: https://reviews.llvm.org/D141912

Tests relying on this should explicitly use -passes='require<analysis>,foo'.

This reverts commit bda6e5bee04c75b1f1332b4fd1ac4e8ef6c3c247.

See https://lab.llvm.org/buildbot/#/builders/109/builds/15424 for instance