[llvm] [GlobalISel] Add multi-way splitting support for wide scalar shifts. (PR #155353)

[Amara Emerson via llvm-commits]

aemerson wrote:

> In the long run would we want to shift via the stack, like DAG does?

I'd rather we do that only as a last resort. Although it's terrible either way, once we legalize through the stack there's no more optimization opportunities.

https://github.com/llvm/llvm-project/pull/155353