RequirementMachine: Delete loops that don't have rules in empty context

When a rule is replaced by a rewrite path, if the rule is in context
then every step of the replacement step is necessarily always in context.

The only way new rules in empty context can be introduced by a
replacement is if the original rewrite step being replaced had no
context.

Therefore, loops that do not contain rules in empty context will
never provide any additional information during homotopy reduction.

Author: slavapestov

lib/AST/RequirementMachine/HomotopyReduction.cpp

@@ -384,13 +384,18 @@ findRuleToDelete(const llvm::DenseSet<unsigned> *redundantConformances,
                  RewritePath &replacementPath) {
   SmallVector<std::pair<unsigned, unsigned>, 2> redundancyCandidates;
   for (unsigned loopID : indices(Loops)) {
-    const auto &loop = Loops[loopID];
+    auto &loop = Loops[loopID];
     if (loop.isDeleted())
       continue;
 
+    bool foundAny = false;
     for (unsigned ruleID : loop.findRulesAppearingOnceInEmptyContext(*this)) {
       redundancyCandidates.emplace_back(loopID, ruleID);
+      foundAny = true;
     }
+
+    if (!foundAny)
+      loop.markDeleted();
   }
 
   Optional<std::pair<unsigned, unsigned>> found;