[llvm] [LV] Add initial support for vectorizing literal struct return values (PR #109833)

[Benjamin Maxwell via llvm-commits]

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MacDue wrote:

I've added `@scalarized_predicated_struct_return` here as a simple test to ensure `computePredInstDiscount` does something sensible for struct returns. 

https://github.com/llvm/llvm-project/pull/109833