[NFC]Add assert to avoid deref nullptr

[XinWang10]

Summary

Static analyzer reports dyn_cast at line 970 could return nullptr which could lead to security issue when deref it at line 971.
I think author could confirm the ptr Code could always be StringInit type here so I add assert to avoid static analyzer report.

[Pierre-vh]

Thanks for the fix! It's good to commit as-is if you want a quick fix, but the version I propose is the best possible user experience.
Note it's all going away in D156315 anyway.

llvm/utils/TableGen/GlobalISelCombinerMatchTableEmitter.cpp

970–973

Ideally do this, otherwise just change the dyn_cast to a cast and it'll just assert if it's the wrong type.

[XinWang10]

OK, Thanks for explanation. Since it will be a big change in D156315 and this issue could disappear, I would abandon this unnecessary revision.

llvm/utils/TableGen/GlobalISelCombinerMatchTableEmitter.cpp

if (Apply.getOperatorAsDef(RuleDef.getLoc())->getName() != "apply") {
     return false;
   }
 
   if (Apply.getNumArgs() != 1) {
     PrintError("Expected exactly 1 argument in 'apply'");
     return false;
   }
 
   const StringInit *Code = dyn_cast<StringInit>(Apply.getArg(0));
+  assert(Code && "Expected 1 StringInit argument in 'apply'");
   auto Pat = std::make_unique<CXXPattern>(*Code, makeAnonPatName("apply"),
                                           /*IsApply*/ true);
   ApplyPats[Pat->getName()] = std::move(Pat);
   return true;
 }
 
 std::unique_ptr<Pattern>
 CombineRuleBuilder::parseInstructionMatcher(const Init &Arg, StringRef Name) {
   const DagInit *Matcher = getDagWithOperatorOfSubClass(Arg, "Instruction");
   if (!Matcher)