[llvm] Complex deinterleaving/single reductions build fix Reapply "Add support for single reductions in ComplexDeinterleavingPass (#112875)" (PR #120441)

[Nicholas Guy via llvm-commits]

NickGuy-Arm wrote:

> Please add in the PR description what changed relative to the first PR. If it continues to cause issues, it will be useful to know that, especially if it's months from now.

Done, I'll add a test for this case shortly too

https://github.com/llvm/llvm-project/pull/120441