Propagating "noalias" metadata for LLVM has actually finally been enabled again recently in nightly [0].
However it has already caused some regressions so it is not clear whether we may go through another revert/fix in llvm/reenable cycle [1]. This has happened several times already sadly [2] as, exactly as you say, basically nobody else has forged through these paths in LLVM before.
[0]: https://github.com/rust-lang/rust/pull/82834
[1]: https://github.com/rust-lang/rust/issues/84958
[2]: https://stackoverflow.com/a/57259339