[llvm] [AMDGPU][True16][Codegen] remove another build_vector pattern from true16 (PR #149861)

[Joe Nash via llvm-commits]

https://github.com/Sisyph commented:

Its not obvious from the test changes why this was illegal. May need a specific test that fails to compile without this patch.

https://github.com/llvm/llvm-project/pull/149861