[llvm] [NFC][TableGen] Adopt `CodeGenInstruction::getName()` (PR #156968)

[Matt Arsenault via llvm-commits]

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@@ -451,7 +450,7 @@ void AsmWriterEmitter::EmitGetMnemonic(
       << "[] = {\n";
     for (unsigned i = 0, e = NumberedInstructions.size(); i != e; ++i) {
       O << "    " << ((OpcodeInfo[i] >> Shift) & Mask) << "U,\t// "
-        << NumberedInstructions[i]->TheDef->getName() << "\n";
+        << NumberedInstructions[i]->getName() << "\n";
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arsenm wrote:

```suggestion
        << NumberedInstructions[i]->getName() << '\n';
```

https://github.com/llvm/llvm-project/pull/156968