[llvm] sext/zext instcombine (PR #67895)

[Nikita Popov via llvm-commits]

https://github.com/nikic commented:

As a high level comment, I'm not a big fan of this long list of special cases. An alternative approach to this would to construct the truth table for this explicitly (i.e. evaluate `icmp pred (ext1 x), (ext2 y)` explicitly for all 4 x, y) and then use a helper that converts a binary truth table into logic.

Unfortunately, I don't think we have an existing helper to convert a truth table to logic (I think @goldsteinn wrote something for vternlog, which is not quite what we'd need here).

https://github.com/llvm/llvm-project/pull/67895