[llvm] [VectorCombine] Fold deinterleave/interleave pairs (PR #211022)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> Thanks @huntergr-arm for pointing to it. Can we handle this? https://godbolt.org/z/fTY3Mn1sv It helps even when interleave does not cancels deinterleave. Once deinterleave and interleave are near instsimplify will clean it up anyway.

This combine currently only (conservatively) applies then both the interleave/deinterleave can be eliminated. Though I don't think adding support handling cases that still need the deinterleave is much of an extension (though we don't have any motivating examples where that's required).      

https://github.com/llvm/llvm-project/pull/211022