[llvm] [LoopVectorizer][ARM] Detect reduce(ext(mul(ext, ext))) patterns more reliably (PR #115847)

[David Green via llvm-commits]

davemgreen wrote:

@ElvisWang123 if this makes the cost model change easier or more difficult let me know. It should just bring the two cost-models closer together, but the new costmodel will get the benefit already. We can easily drop this if its simpler that way.

https://github.com/llvm/llvm-project/pull/115847