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

For instructions that don't map to a mnemonic string, the implementation
of MCInstPrinter::getMnemonic would return an invalid pointer due to the
result of the calculation of the instruction's position in the `AsmStrs`
table. This patch fixes the issue by ensuring those cases return a
`nullptr` value instead.

Fixes llvm#74177.

Author: pratlucas

llvm/lib/MC/MCAsmStreamer.cpp

@@ -153,7 +153,10 @@ class MCAsmStreamer final : public MCStreamer {
   void emitGNUAttribute(unsigned Tag, unsigned Value) override;
 
   StringRef getMnemonic(MCInst &MI) override {
-    return InstPrinter->getMnemonic(&MI).first;
+    auto [Ptr, Bits] = InstPrinter->getMnemonic(&MI);
+    assert((Bits != 0 || Ptr == nullptr) &&
+           "Invalid char pointer for instruction with no mnemonic");
+    return Ptr;
   }
 
   void emitLabel(MCSymbol *Symbol, SMLoc Loc = SMLoc()) override;

llvm/utils/TableGen/AsmWriterEmitter.cpp

@@ -437,6 +437,10 @@ void AsmWriterEmitter::EmitGetMnemonic(
   O << "  // Emit the opcode for the instruction.\n";
   O << BitsString;
 
+  // Make sure we don't return an invalid pointer if bits is 0
+  O << "  if (Bits == 0)\n"
+       "    return {nullptr, Bits};\n";
+
   // Return mnemonic string and bits.
   O << "  return {AsmStrs+(Bits & " << (1 << AsmStrBits) - 1
     << ")-1, Bits};\n\n";