[IR] Mark assume/annotation as InaccessibleMemOnly

[IR] Mark assume/annotation as InaccessibleMemOnly

These intrinsics don't need to be marked as arbitrary writing,
it's sufficient to write inaccessible memory (aka "side effect")
to preserve control dependencies. This means less special-casing
in BasicAA. This is intended as an alternative to D98925.

Differential Revision: https://reviews.llvm.org/D99022