[TableGen] Remove duplicate code in applyMnemonicAliases when target uses DefaultAsmParserVariant. (#108494)

The DefaultAsmParserVariant has an empty name. MnemonicAlias uses an
empty string to mean the alias applies to all variants.

Targets that uses DefaultAsmParserVariant were emitting the same code
inside the variant loop and after the variant loop because an empty
string got passed to emitMnemonicAliasVariant in both places.

This patch detects the empty variant name in the loop and skips the
emission.

Author: topperc

llvm/utils/TableGen/AsmMatcherEmitter.cpp

@@ -2849,18 +2849,29 @@ static bool emitMnemonicAliases(raw_ostream &OS, const AsmMatcherInfo &Info,
 
   OS << "static void applyMnemonicAliases(StringRef &Mnemonic, "
         "const FeatureBitset &Features, unsigned VariantID) {\n";
-  OS << "  switch (VariantID) {\n";
   unsigned VariantCount = Target.getAsmParserVariantCount();
   for (unsigned VC = 0; VC != VariantCount; ++VC) {
     Record *AsmVariant = Target.getAsmParserVariant(VC);
     int AsmParserVariantNo = AsmVariant->getValueAsInt("Variant");
     StringRef AsmParserVariantName = AsmVariant->getValueAsString("Name");
+
+    // If the variant doesn't have a name, defer to the emitMnemonicAliasVariant
+    // call after the loop.
+    if (AsmParserVariantName.empty()) {
+      assert(VariantCount == 1 && "Multiple variants should each be named");
+      continue;
+    }
+
+    if (VC == 0)
+      OS << "  switch (VariantID) {\n";
     OS << "  case " << AsmParserVariantNo << ":\n";
     emitMnemonicAliasVariant(OS, Info, Aliases, /*Indent=*/2,
                              AsmParserVariantName);
     OS << "    break;\n";
+
+    if (VC == VariantCount - 1)
+      OS << "  }\n";
   }
-  OS << "  }\n";
 
   // Emit aliases that apply to all variants.
   emitMnemonicAliasVariant(OS, Info, Aliases);