[RISCV] Remove an extra map lookup from RISCVCompressInstEmitter. NFC

When we looked up the map to see if the entry already existed,
this created the new entry for us. So save a reference to it so
we can use it to update the entry instead of looking it up again.

Also remove unnecessary StringRef constructors around string
literals on calls to this function.

llvm/utils/TableGen/RISCVCompressInstEmitter.cpp

@@ -509,14 +509,13 @@ getReqFeatures(std::set<std::pair<bool, StringRef>> &FeaturesSet,
 static unsigned getPredicates(DenseMap<const Record *, unsigned> &PredicateMap,
                               std::vector<const Record *> &Predicates,
                               Record *Rec, StringRef Name) {
-  unsigned Entry = PredicateMap[Rec];
+  unsigned &Entry = PredicateMap[Rec];
   if (Entry)
     return Entry;
 
   if (!Rec->isValueUnset(Name)) {
     Predicates.push_back(Rec);
     Entry = Predicates.size();
-    PredicateMap[Rec] = Entry;
     return Entry;
   }
 
@@ -751,14 +750,16 @@ void RISCVCompressInstEmitter::emitCompressInstEmitter(raw_ostream &o,
         } else {
           // Handling immediate operands.
           if (CompressOrUncompress) {
-            unsigned Entry = getPredicates(MCOpPredicateMap, MCOpPredicates,
-              DestOperand.Rec, StringRef("MCOperandPredicate"));
+            unsigned Entry =
+                getPredicates(MCOpPredicateMap, MCOpPredicates, DestOperand.Rec,
+                              "MCOperandPredicate");
             CondStream.indent(6)
                 << Namespace << "ValidateMCOperand("
                 << "MI.getOperand(" << OpIdx << "), STI, " << Entry << ") &&\n";
           } else {
-            unsigned Entry = getPredicates(ImmLeafPredicateMap, ImmLeafPredicates,
-              DestOperand.Rec, StringRef("ImmediateCode"));
+            unsigned Entry =
+                getPredicates(ImmLeafPredicateMap, ImmLeafPredicates,
+                              DestOperand.Rec, "ImmediateCode");
             CondStream.indent(6)
                 << "MI.getOperand(" << OpIdx << ").isImm() &&\n";
             CondStream.indent(6) << Namespace << "ValidateMachineOperand("
@@ -774,14 +775,14 @@ void RISCVCompressInstEmitter::emitCompressInstEmitter(raw_ostream &o,
       case OpData::Imm: {
         if (CompressOrUncompress) {
           unsigned Entry = getPredicates(MCOpPredicateMap, MCOpPredicates,
-            DestOperand.Rec, StringRef("MCOperandPredicate"));
+                                         DestOperand.Rec, "MCOperandPredicate");
           CondStream.indent(6)
               << Namespace << "ValidateMCOperand("
               << "MCOperand::createImm(" << DestOperandMap[OpNo].Data.Imm
               << "), STI, " << Entry << ") &&\n";
         } else {
           unsigned Entry = getPredicates(ImmLeafPredicateMap, ImmLeafPredicates,
-            DestOperand.Rec, StringRef("ImmediateCode"));
+                                         DestOperand.Rec, "ImmediateCode");
           CondStream.indent(6)
               << Namespace
               << "ValidateMachineOperand(MachineOperand::CreateImm("