Mark begin_access is non-clonable (!isTriviallyDuplicatable()).

In one shot, this prevents multiple CFG transforms from breaking the simple
begin/end access marker pattern for access scopes.

Author: atrick

lib/SIL/LoopInfo.cpp

@@ -81,6 +81,9 @@ bool SILLoop::canDuplicate(SILInstruction *I) const {
   if (isa<ThrowInst>(I))
     return false;
 
+  if (isa<BeginAccessInst>(I))
+    return false;
+
   assert(I->isTriviallyDuplicatable() &&
     "Code here must match isTriviallyDuplicatable in SILInstruction");
   return true;

lib/SIL/SILInstruction.cpp

@@ -1142,17 +1142,13 @@ SILInstruction *SILInstruction::clone(SILInstruction *InsertPt) {
 /// additional handling. It is important to know this information when
 /// you perform such optimizations like e.g. jump-threading.
 bool SILInstruction::isTriviallyDuplicatable() const {
-  if (isa<ThrowInst>(this))
-    return false;
-
   if (isa<AllocStackInst>(this) || isa<DeallocStackInst>(this)) {
     return false;
   }
   if (auto *ARI = dyn_cast<AllocRefInst>(this)) {
     if (ARI->canAllocOnStack())
       return false;
   }
-
   if (isa<OpenExistentialAddrInst>(this) || isa<OpenExistentialRefInst>(this) ||
       isa<OpenExistentialMetatypeInst>(this) ||
       isa<OpenExistentialValueInst>(this) || isa<OpenExistentialBoxInst>(this) ||
@@ -1169,6 +1165,13 @@ bool SILInstruction::isTriviallyDuplicatable() const {
     if (MI->getMember().isForeign)
       return false;
   }
+  if (isa<ThrowInst>(this))
+    return false;
+
+  // BeginAccess defines the access scope entry point. All associated EndAccess
+  // instructions must directly operate on the BeginAccess.
+  if (isa<BeginAccessInst>(this))
+    return false;
 
   return true;
 }