[llvm] [AArch64] Optimize vector multiplications by certain constants for v2i64 (PR #183827)

[Aly ElAshram via llvm-commits]

AlyElashram wrote:

> Is that worth doing too? It would be less than the 6 instructions needed to scalarize.
I think it is worth doing. I misunderstood your comment , I thought you meant just implementing the optimization in a `decomposeMulByConstant` function so we can expand on it later. 
Taking a look at this tomorrow again to implement the missing optimization 👀 

https://github.com/llvm/llvm-project/pull/183827