[X86][NFC] MnemonicTables: only access RI fields if they're initialized

[Amir]

Summary

Fix an issue reported by UBSan.

Test Plan:
Configure with -DLLVM_USE_SANITIZER="Address;Undefined"
ninja llc

[skan]

Looks reasonable.

llvm/utils/TableGen/X86MnemonicTables.cpp

void X86MnemonicTablesEmitter::run(raw_ostream &OS) {
   // Unused
   X86Disassembler::DisassemblerTables Tables;
   ArrayRef<const CodeGenInstruction *> NumberedInstructions =
       Target.getInstructionsByEnumValue();
   for (unsigned II = 0, IE = NumberedInstructions.size(); II != IE; ++II) {
     const CodeGenInstruction *I = NumberedInstructions[II];
     X86Disassembler::RecognizableInstr RI(Tables, *I, II);
     Record *Def = I->TheDef;
-    bool IsCodeGenOnly = RI.IsCodeGenOnly;
-    bool ForceDisassemble = RI.ForceDisassemble;
-    uint8_t Form = RI.Form;
     if ( // Filter non-X86 instructions
         !Def->isSubClassOf("X86Inst") ||
         // Skip pseudo instructions as they may contain non-alnum characters in
         // mnemonic
-        (IsCodeGenOnly && !ForceDisassemble) ||
+        (RI.IsCodeGenOnly && !RI.ForceDisassemble) ||
         // Non-parsable instruction defs contain prefix as part of AsmString
         Def->getValueAsString("AsmVariantName") == "NonParsable" ||
         // Skip CodeGenInstructions that are not real standalone instructions
-        Form == X86Local::PrefixByte || Form == X86Local::Pseudo)
+        RI.Form == X86Local::PrefixByte || RI.Form == X86Local::Pseudo)
       continue;
     // Flatten an instruction assembly string.
     std::string AsmString = I->FlattenAsmStringVariants(I->AsmString, Variant);
     StringRef Mnemonic(AsmString);
     // Extract a mnemonic assuming it's separated by \t
     Mnemonic = Mnemonic.take_until([](char C) { return C == '\t'; });
 
     // Special case: CMOVCC, JCC, SETCC have "${cond}" in mnemonic.