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

[Shilei Tian 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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shiltian wrote:

Is the GIsel version in a separate file with name `-gisel` or something? I'm wondering maybe we can merge them?

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