Redo the data flow part of AccessEnforcementOpts.

Directly implement the data flow. Eliminate the extraneous work.
Remove cubic behavior. Do a single iteration of the data flow state at
each program point only performing the necessary set operations. At
unidentified access, clear the sets for simplicity and efficiency.

This cleanup results in significant functional changes:

- Allowing scopes to merge even if they are enclosed.

- Handling unidentified access conservatively.

Note that most of the added lines of code are comments.

Somehow this cleanup incidentally fixes:
<rdar://problem/48514339> swift compiler hangs building project

(I expected the subsequent loop summary cleanup to fix that problem.)

Author: atrick

include/swift/SILOptimizer/Analysis/AccessedStorageAnalysis.h

@@ -122,6 +122,8 @@ template <> struct DenseMapInfo<swift::StorageAccessInfo> {
 }
 
 namespace swift {
+using AccessedStorageSet = llvm::SmallDenseSet<StorageAccessInfo, 8>;
+
 /// Records each unique AccessedStorage in a set of StorageAccessInfo
 /// objects. Hashing and equality only sees the AccesedStorage data. The
 /// additional StorageAccessInfo bits are recorded as results of this analysis.
@@ -132,17 +134,17 @@ namespace swift {
 /// large. It does not imply that all accesses have Unidentified
 /// AccessedStorage, which is never allowed for class or global access.
 class AccessedStorageResult {
-
-  using AccessedStorageSet = llvm::SmallDenseSet<StorageAccessInfo, 8>;
-
   AccessedStorageSet storageAccessSet;
   Optional<SILAccessKind> unidentifiedAccess;
+
 public:
   AccessedStorageResult() {}
 
   // ---------------------------------------------------------------------------
   // Accessing the results.
 
+  const AccessedStorageSet &getStorageSet() const { return storageAccessSet; }
+
   bool isEmpty() const {
     return storageAccessSet.empty() && !unidentifiedAccess;
   }