[llvm] [WebAssembly] Add support for avgr_u in loops (PR #153252)

[Luke Lau via llvm-commits]

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lukel97 wrote:

I don't think we need the entire loop vectorizer output here. Can we instead extend the existing `avgr_u` tests in `simd-arith.ll` to also have `avgr_u_lshr` versions? Where instead of the udiv we have lshr

https://github.com/llvm/llvm-project/pull/153252