[llvm] Add pre-merge workflow for HLSL testing (PR #122184)

[Chris B via llvm-commits]

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llvm-beanz wrote:

Yea, that makes sense. The structuring as it is was to produce status badges:
<img width="832" alt="Screenshot 2025-01-09 at 10 24 58 AM" src="https://github.com/user-attachments/assets/54c0bad4-8a98-4f88-805f-c822451fa748" />
(see: https://github.com/llvm-beanz/offload-test-suite/?tab=readme-ov-file#current-status)

Those require unique workflows at the top level and can't provide matrix granularity.

Obviously this isn't a consideration here, so I can refactor this.

https://github.com/llvm/llvm-project/pull/122184