[TableGen] Report an error instead of asserting

[foad]

Summary

This gives a nice error if you accidentally try to use an empty list for
the RegTypes of a RegisterClass.

[foad]

Ping.

[nhaehnle]

LGTM

[arsenm]

I think you should be able to define regclasses without types, but this is an improvement

llvm/test/TableGen/RegisterClass.td

+// RUN: not llvm-tblgen -gen-register-bank -I %p/../../include %s 2>&1 | FileCheck %s
+
+include "llvm/Target/Target.td"
+
+def MyTarget : Target;
+def R0 : Register<"r0">;
+def ClassA : RegisterClass<"MyTarget", [], 32, (add R0)>; // CHECK: [[@LINE]]:1: error: RegTypes list must not be empty!

llvm/utils/TableGen/CodeGenRegisters.cpp

static void sortAndUniqueRegisters(CodeGenRegister::Vec &M) {
   M.erase(std::unique(M.begin(), M.end(), deref<std::equal_to<>>()), M.end());
 }
 
 CodeGenRegisterClass::CodeGenRegisterClass(CodeGenRegBank &RegBank, Record *R)
     : TheDef(R), Name(std::string(R->getName())),
       TopoSigs(RegBank.getNumTopoSigs()), EnumValue(-1) {
   GeneratePressureSet = R->getValueAsBit("GeneratePressureSet");
   std::vector<Record*> TypeList = R->getValueAsListOfDefs("RegTypes");
+  if (TypeList.empty())
+    PrintFatalError(R->getLoc(), "RegTypes list must not be empty!");
   for (unsigned i = 0, e = TypeList.size(); i != e; ++i) {
     Record *Type = TypeList[i];
     if (!Type->isSubClassOf("ValueType"))
       PrintFatalError(R->getLoc(),
                       "RegTypes list member '" + Type->getName() +
                           "' does not derive from the ValueType class!");
     VTs.push_back(getValueTypeByHwMode(Type, RegBank.getHwModes()));
   }
-  assert(!VTs.empty() && "RegisterClass must contain at least one ValueType!");
 
   // Allocation order 0 is the full set. AltOrders provides others.
   const SetTheory::RecVec *Elements = RegBank.getSets().expand(R);
   ListInit *AltOrders = R->getValueAsListInit("AltOrders");
   Orders.resize(1 + AltOrders->size());
 
   // Default allocation order always contains all registers.
   Artificial = true;