[llvm] [LV] Simplify the chain traversal in `getScaledReductions()` (NFCI) (PR #184830)

[Gaëtan Bossu via llvm-commits]

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gbossu wrote:

Nit: Not your doing, but the comment mentions a "valid partial reduction". It would be helpful to document which valid forms are expected. According to `MatchExtends`, I think we expect:
* either `UpdateR( BinOp(ext(...), ext(...)), Phi )`
* or `UpdateR( ext(...), Phi )`

And on top of that, we add another layer here to match `UpdateR( ext(BinOp(ext(...), ext(...))), Phi )`. But this new ext isn't added to `CastRecipes` for some reason. I think it would help to match the simple  `UpdateR( ext(...), Phi )` case first and have an early exit. In another PR if you agree :)

https://github.com/llvm/llvm-project/pull/184830