[llvm] [SelectionDAG] Merge consecutive loads feeding as shuffle operands (PR #207303)

[Paul Walker via llvm-commits]

paulwalker-arm wrote:

> > Please split the PR into two commits with the first having only the new tests. Doing this makes it much simpler to understand the effects of the code changes.
> 
> Done. PR #207368

For clarity I didn't mean a separate PR. I'm fine with keeping all the change in a single PR, but having it spread across two commits means you can see the effects of the code changes by select only those commits when reviewing.

https://github.com/llvm/llvm-project/pull/207303