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

[Matt Arsenault via llvm-commits]

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arsenm wrote:

At this point we are over-using MIR tests which are bigger, less maintainable, and test less. They are useful for testing edge cases that are difficult to reproduce, but simple stuff like this are better off just using IR. MIR legalizer tests were more useful at the start when we couldn't run as much code through end to end 

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