[llvm] [LV] Bundle partial reductions inside VPExpressionRecipe (PR #147302)

[Sander de Smalen via llvm-commits]

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sdesmalen-arm wrote:

The case where `ExtBR` is a nullptr is the only case that I think needs handling (which already seems to be tried below on line 347?)

https://github.com/llvm/llvm-project/pull/147302