[TableGen] Print message about dropped patterns with -debug

A selection pattern can be silently dropped if type inference fails to
deduce some types. Print a message when that happens, and -debug was
applied.

Author: Krzysztof Parzyszek

llvm/utils/TableGen/CodeGenDAGPatterns.cpp

@@ -4333,13 +4333,24 @@ void CodeGenDAGPatterns::ParseOnePattern(Record *TheDef,
   // that register class does not accept that type, the type inference
   // will lead to a contradiction, which is not an error however, but
   // a sign that this pattern will simply never match.
-  if (Temp.getOnlyTree()->hasPossibleType())
-    for (const auto &T : Pattern.getTrees())
+  if (Temp.getOnlyTree()->hasPossibleType()) {
+    for (const auto &T : Pattern.getTrees()) {
       if (T->hasPossibleType())
         AddPatternToMatch(&Pattern,
                           PatternToMatch(TheDef, Preds, T, Temp.getOnlyTree(),
                                          InstImpResults, Complexity,
                                          TheDef->getID()));
+    }
+  } else {
+    // Show a message about a dropped pattern with some info to make it
+    // easier to identify it in the .td files.
+    LLVM_DEBUG({
+      dbgs() << "Dropping: ";
+      Pattern.dump();
+      Temp.getOnlyTree()->dump();
+      dbgs() << "\n";
+    });
+  }
 }
 
 void CodeGenDAGPatterns::ParsePatterns() {