[llvm] [GlobalISel] Fix buildCopyFromRegs for split vectors (PR #77448)

[Matt Arsenault via llvm-commits]

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@@ -0,0 +1,13790 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
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arsenm wrote:

I think the test should be restricted to argument / return values with bfloats, and check the generated IR translator MIR. Don't copy the full bf16 test, that can just add -global-isel run lines 

https://github.com/llvm/llvm-project/pull/77448