[llvm] [AMDGPU] Correct bitshift legality transformation for small vectors (PR #140940)

[Shilei Tian via llvm-commits]

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shiltian wrote:

The benefit of using MIR here is to be able to get auto generate check lines. Maybe we can teach the script to be able to generate check lines for llc test with IR input and MIR output.

https://github.com/llvm/llvm-project/pull/140940