[MCA] Avoid repeated hash lookups (NFC) (#129192)

Author: kazutakahirata

llvm/lib/MCA/InstrBuilder.cpp

@@ -634,16 +634,14 @@ InstrBuilder::createInstrDescImpl(const MCInst &MCI,
   bool IsVariadic = MCDesc.isVariadic();
   if ((ID->IsRecyclable = !IsVariadic && !IsVariant)) {
     auto DKey = std::make_pair(MCI.getOpcode(), SchedClassID);
-    Descriptors[DKey] = std::move(ID);
-    return *Descriptors[DKey];
+    return *(Descriptors[DKey] = std::move(ID));
   }
 
   auto VDKey = std::make_pair(hashMCInst(MCI), SchedClassID);
   assert(
       !VariantDescriptors.contains(VDKey) &&
       "Expected VariantDescriptors to not already have a value for this key.");
-  VariantDescriptors[VDKey] = std::move(ID);
-  return *VariantDescriptors[VDKey];
+  return *(VariantDescriptors[VDKey] = std::move(ID));
 }
 
 Expected<const InstrDesc &>