[CodeGenDAGPatterns] Replaces some implicit map inserts with count.

[staronj]

Summary

Working on clang-tidy check that is finding modifications of container you are iterating I found this lines of code. Here, we are using operator[] on std::map and as I understand, depending on the behaviour of operator[] - creating new entry in absence of key (ie. as I understand, comparisons in line 3223 and 3231 should succeed only if Entry.first was not in map).

Changing it to explicitly checking presence in map is cleaner and less bug-prone.

[Prazek]

LGTM, but I would wait for someone else to accept it.

utils/TableGen/CodeGenDAGPatterns.cpp

void CodeGenDAGPatterns::AddPatternToMatch(TreePattern *Pattern,
   // same type.
   std::map<std::string, NameRecord> SrcNames, DstNames;
   FindNames(PTM.getSrcPattern(), SrcNames, Pattern);
   FindNames(PTM.getDstPattern(), DstNames, Pattern);
 
   // Scan all of the named values in the destination pattern, rejecting them if
   // they don't exist in the input pattern.
   for (const auto &Entry : DstNames) {
-    if (SrcNames[Entry.first].first == nullptr)
+    if (SrcNames.count(Entry.first) == 0)
       Pattern->error("Pattern has input without matching name in output: $" +
                      Entry.first);
   }
 
   // Scan all of the named values in the source pattern, rejecting them if the
   // name isn't used in the dest, and isn't used to tie two values together.
   for (const auto &Entry : SrcNames)
-    if (DstNames[Entry.first].first == nullptr &&
-        SrcNames[Entry.first].second == 1)
+    if (DstNames.count(Entry.first) == 0 &&
+        Entry.second.second == 1)
       Pattern->error("Pattern has dead named input: $" + Entry.first);
 
   PatternsToMatch.push_back(PTM);
 }
 
 
 
 void CodeGenDAGPatterns::InferInstructionFlags() {