[llvm] [X86][Codegen] Shuffle certain shifts on i8 vectors to create opportunity for vectorized shift instructions (PR #117980)

[William Huang via llvm-commits]

huangjd wrote:

> Depending on your target CPU I did start work on generic vXi8 shift lowering using GFNI instructions: https://github.com/llvm/llvm-project/pull/89644 which I haven't had time to go back to, have you looked at anything similar?

Won't affect GFNI, as my patch does not apply on AVX512, and all GFNI CPU has AVX512

https://github.com/llvm/llvm-project/pull/117980