[llvm] [VFABI] Add support for vector functions that return struct types (PR #119000)

[David Sherwood via llvm-commits]

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david-arm wrote:

I think it might be worth renaming this StructWideningUtils.cpp to better reflect what's happening. I realise the only use case so far is for vectorising functions that return a struct, but in principle it could be used for other things too.

https://github.com/llvm/llvm-project/pull/119000