GlobalISel: Add debug print for applied rule in generated combiner

[dsanders]

LGTM

[arsenm]

a583654a8b1767190e08f2ec568841c6aaad4e3c

llvm/utils/TableGen/GICombinerEmitter.cpp

if (Rule->getMatchingFixupCode() &&
       // before the ruleset grows to small-moderate size. Last time, it became
       // a big problem for low-mem systems around the 500 rule mark but by the
       // time we grow that large we should have merged the ISel match table
       // mechanism with the Combiner.
       OS << Indent << "      && [&]() {\n"
          << Indent << "      "
          << CodeExpander(Rule->getMatchingFixupCode()->getValue(), Expansions,
                          RuleDef.getLoc(), ShowExpansions)
-         << "\n"
+         << '\n'
          << Indent << "      return true;\n"
          << Indent << "  }()";
     }
     OS << Indent << "     ) {\n" << Indent << "   ";
 
     if (const StringInit *Code = dyn_cast<StringInit>(Applyer->getArg(0))) {
-      OS << CodeExpander(Code->getAsUnquotedString(), Expansions,
+      OS << "    LLVM_DEBUG(dbgs() << \"Applying rule '"
+         << RuleDef.getName()
+         << "'\\n\");\n"
+         << CodeExpander(Code->getAsUnquotedString(), Expansions,
                          RuleDef.getLoc(), ShowExpansions)
-         << "\n"
+         << '\n'
          << Indent << "    return true;\n"
          << Indent << "  }\n";
     } else {
       PrintError(RuleDef.getLoc(), "Expected apply code block");
       return;
     }
 
     OS << Indent << "}\n";