[clang] [llvm] [mlir] Enforce single-operand form for llvm.loop.distribute metadata (PR #201077)

[Michael Kruse via cfe-commits]

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Meinersbur wrote:

[not a change request] `CHECK-NOT` tests are not reliable. Here, if the `!{!"llvm.loop.distribute.disable"}` matches, the `-NOT` will only check after the line. That is, the following will not cause an error:

```
!1 = !{!"llvm.loop.distribute.enable"}
...
!2 = !{!"llvm.loop.distribute.enable", i1 true}
...
!3 = !{!"llvm.loop.distribute.disable"}
```

I'd avoid relying on `-NOT` tests, but they will also not hurt here either an may make the intention of this test clear.

https://github.com/llvm/llvm-project/pull/201077