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

[Min-Yih Hsu via llvm-commits]

mshockwave wrote:

> What is illegal here? It tries to emit long vectors, relying on backend (for now) on splitting.

`<19 x i16>` is considered an illegal type for the RISCV backend and since we need to spend extra instructions to split it (as you also mentioned), I vaguely remember loop vectorizer would give it a higher cost and thus avoiding such types. But maybe that's just LV.

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