[clang] [CIR] Upstream initial support for CIR flattening (PR #130648)

[Erich Keane via cfe-commits]

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@@ -603,6 +606,64 @@ static void prepareTypeConverter(mlir::LLVMTypeConverter &converter,
   });
 }
 
+// The unreachable code is not lowered by applyPartialConversion function
+// since it traverses blocks in the dominance order. At the same time we
+// do need to lower such code - otherwise verification errors occur.
+// For instance, the next CIR code:
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erichkeane wrote:

```suggestion
// For instance, this CIR code:
```

https://github.com/llvm/llvm-project/pull/130648