[llvm] [TableGen] Use MVT instead of MVT::SimpleValueType. NFC (PR #169180)

[Sergei Barannikov via llvm-commits]

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@@ -41,13 +41,13 @@ static cl::opt<unsigned>
 
 /// getValueType - Return the MVT::SimpleValueType that the specified TableGen
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s-barannikov wrote:

```suggestion
/// Returns the MVT that the specified TableGen
```

https://github.com/llvm/llvm-project/pull/169180