[DebugInfo] Fix undef debug values being removed

Author: Snowy1803

lib/IRGen/IRGenSIL.cpp

@@ -5598,11 +5598,10 @@ void IRGenSILFunction::visitDebugValueInst(DebugValueInst *i) {
 
   auto VarInfo = i->getVarInfo();
   assert(VarInfo && "debug_value without debug info");
-  if (isa<SILUndef>(SILVal)) {
+  if (isa<SILUndef>(SILVal) && VarInfo->Name == "$error") {
     // We cannot track the location of inlined error arguments because it has no
     // representation in SIL.
-    if (!IsAddrVal &&
-        !i->getDebugScope()->InlinedCallSite && VarInfo->Name == "$error") {
+    if (!IsAddrVal && !i->getDebugScope()->InlinedCallSite) {
       auto funcTy = CurSILFn->getLoweredFunctionType();
       emitErrorResultVar(funcTy, funcTy->getErrorResult(), i);
     }

lib/SILOptimizer/Transforms/DeadCodeElimination.cpp

@@ -77,8 +77,10 @@ static bool seemsUseful(SILInstruction *I) {
   }
 
   // Is useful if it's associating with a function argument
+  // If undef, it is useful and it doesn't cost anything.
   if (isa<DebugValueInst>(I))
-    return isa<SILFunctionArgument>(I->getOperand(0));
+    return isa<SILFunctionArgument>(I->getOperand(0))
+      || isa<SILUndef>(I->getOperand(0));
 
   return false;
 }