[llvm] [SLP]Support LShr as base for copyable elements (PR #153393)

[Min-Yih Hsu via llvm-commits]

mshockwave wrote:

While the DAGCombiner was wrong, I have a meta question for @alexey-bataev: is it expected for SLP to generate `<19 x i16>` in the first place? I thought it would try to avoid illegal types.

https://github.com/llvm/llvm-project/pull/153393