[SILDiagnostics] Fix an invalidation crash in DiagnoseStaticExclusivity.

Fix a crash when re-hashing the basic-block map would leave behind a dangling
mutable reference.

The irony that the language feature this pass implements would have caught this
issue statically is not lost on me.

Author: devincoughlin

lib/SILOptimizer/Mandatory/DiagnoseStaticExclusivity.cpp

@@ -295,7 +295,11 @@ static void checkStaticExclusivity(SILFunction &Fn) {
     // state for that predecessor as our in state. The SIL verifier guarantees
     // that all incoming edges must have the same current accesses.
     for (auto *Pred : BB->getPredecessorBlocks()) {
-      const Optional<StorageMap> &PredAccesses = BlockOutAccesses[Pred];
+      auto it = BlockOutAccesses.find(Pred);
+      if (it == BlockOutAccesses.end())
+        continue;
+
+      const Optional<StorageMap> &PredAccesses = it->getSecond();
       if (PredAccesses) {
         BBState = PredAccesses;
         break;

test/SILOptimizer/exclusivity_static_diagnostics.sil

@@ -341,3 +341,64 @@ bb0(%0 : $Int):
   %9 = tuple ()
   return %9 : $()
 }
+
+// Check for iterator invalidation issues when the hash from
+// basic blocks to analysis state is re-hashed. The number of
+// blocks in this test is determined by the initial size of the
+// 'BlockOutAccesses' DenseMap in the implementation.
+//
+//  The unreachable block below must branch to bN where
+//    N = 3/4 * INITIAL_SIZE - 2
+sil @blockMapRehash : $@convention(method) (Builtin.Int1) -> () {
+bb0(%0: $Builtin.Int1):
+ br bb1
+bb1:
+  br bb2
+bb2:
+  br bb3
+bb3:
+  br bb4
+bb4:
+  br bb5
+bb5:
+  br bb6
+bb6:
+  br bb7
+bb7:
+  br bb8
+bb8:
+  br bb9
+bb9:
+  br bb10
+bb10:
+  br bb11
+bb11:
+  br bb12
+bb12:
+  br bb13
+bb13:
+  br bb14
+bb14:
+  br bb15
+bb15:
+  br bb16
+bb16:
+   br bb17
+bb17:
+  br bb18
+bb18:
+  br bb19
+bb19:
+  br bb20
+bb20:
+  br bb21
+bb21:
+  br bb22
+bb22:
+  br bb23 // no-crash
+bb23:
+  %1 = tuple ()
+  return %1 : $()
+bbUnreachable:
+  br bb22
+}