RequirementMachine: Don't record no-op PrefixSubstitutions rewrite steps

Author: slavapestov

lib/AST/RequirementMachine/PropertyMap.cpp

@@ -478,9 +478,11 @@ PropertyMap::concretelySimplifySubstitutions(Term baseTerm, Symbol symbol,
                                                 /*ruleID=*/*props->ConcreteTypeRule,
                                                 /*inverse=*/true));
 
-          path->add(RewriteStep::forPrefixSubstitutions(/*length=*/prefix.size(),
-                                                        /*endOffset=*/0,
-                                                        /*inverse=*/false));
+          if (!prefix.empty()) {
+            path->add(RewriteStep::forPrefixSubstitutions(/*length=*/prefix.size(),
+                                                          /*endOffset=*/0,
+                                                          /*inverse=*/false));
+          }
         }
       }
     }

lib/AST/RequirementMachine/RewriteLoop.cpp

@@ -295,6 +295,8 @@ RewritePathEvaluator::applyRewriteRule(const RewriteStep &step,
 std::pair<MutableTerm, MutableTerm>
 RewritePathEvaluator::applyPrefixSubstitutions(const RewriteStep &step,
                                                const RewriteSystem &system) {
+  assert(step.Arg != 0);
+
   auto &term = getCurrentTerm();
 
   assert(step.Kind == RewriteStep::PrefixSubstitutions);