[llvm] [AsmWriter] Ensure getMnemonic doesn't return invalid pointers (PR #75783)

[Tomas Matheson via llvm-commits]

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@@ -154,7 +154,10 @@ class MCAsmStreamer final : public MCStreamer {
   void emitGNUAttribute(unsigned Tag, unsigned Value) override;
 
   StringRef getMnemonic(MCInst &MI) override {
-    return InstPrinter->getMnemonic(&MI).first;
+    std::pair<const char *, uint64_t> M = InstPrinter->getMnemonic(&MI);
+    assert((M.second != 0 || M.first == nullptr) &&
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tmatheson-arm wrote:

C++17 structured bindings might make this a bit more readable:
```cpp
auto [ Ptr, Bits ] = InstPrinter->getMnemonic(&MI); 
```

https://github.com/llvm/llvm-project/pull/75783