SILGen: Add a couple of assertions

Author: slavapestov

lib/SILGen/SILGenPoly.cpp

@@ -4483,6 +4483,8 @@ void SILGenFunction::emitProtocolWitness(
                                           ->getCanonicalType());
   }
 
+  assert(!witnessSubstTy->hasError());
+
   if (auto genericFnType = dyn_cast<GenericFunctionType>(reqtSubstTy)) {
     auto forwardingSubs = F.getForwardingSubstitutionMap();
     reqtSubstTy = cast<FunctionType>(genericFnType
@@ -4493,6 +4495,8 @@ void SILGenFunction::emitProtocolWitness(
                                           ->getCanonicalType());
   }
 
+  assert(!reqtSubstTy->hasError());
+
   // Get the lowered type of the witness.
   auto origWitnessFTy = getWitnessFunctionType(getTypeExpansionContext(), SGM,
                                                witness, witnessKind);