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

[Benjamin Maxwell via llvm-commits]

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+; RUN: opt < %s -passes=loop-vectorize,dce,instcombine -force-vector-interleave=1 -S | FileCheck %s --check-prefixes=NEON
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MacDue wrote:

Alright, I've split the fixed-size tests into a target agnostic test, and left the scalable tests under AArch64 :+1: 
Also, added another test to print the VPlan (when widening or replicating struct returns).  

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