[llvm] [NFC][DirectX] Refactor `DXILPrepare`/`DXILTranslateMetadata` (PR #164285)

[Finn Plummer via llvm-commits]

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@@ -10,7 +10,6 @@ entry:
 }
 
 ; CHECK-NOT: !dx.rootsignatures
-; CHECK-NOT: {{^!}}
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inbelic wrote:

This would check that no other metadata was defined in the file. Since `translate-metadata` will also emit the other metadata structures, this check will no longer hold. But we do still correctly check that the root signature metadata is removed.

https://github.com/llvm/llvm-project/pull/164285