[llvm] [InstCombine] Fold integer unpack/repack patterns through ZExt (PR #153583)

[via llvm-commits]

zGoldthorpe wrote:

This PR is based on [Yingwei's suggestion](https://github.com/llvm/llvm-project/pull/147737#discussion_r2202368419) made a while ago on #147737.

The original implementation of PR #147737 is too aggressive and rewrites IR in a harder-to-optimise form, and making it conservative makes it difficult to fold the unpack/repack pattern mentioned above.

https://github.com/llvm/llvm-project/pull/153583