[llvm] [AMDGPU] Add testcases for widening i8/i16 shifts in DAGISel (PR #132379)

[Pierre van Houtryve via llvm-commits]

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+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
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Pierre-vh wrote:

No they're just a few testscases I brought over from GlobalISel and build with CGP i16 widening off to showcase a few regressions left to address.

Now that I think about it, this diff can wait until I have a patch that shows changes to these tests

https://github.com/llvm/llvm-project/pull/132379