[AllocBoxToStack] Drop dead_end dealloc_boxes.

When creating `dealloc_stack`s corresponding to `dealloc_box`es, don't
bother to create a dealloc_stack if the `dealloc_box` is flagged
`dead_end`.

Author: nate-chandler

lib/SILOptimizer/Transforms/AllocBoxToStack.cpp

@@ -709,6 +709,11 @@ static bool rewriteAllocBoxAsAllocStack(AllocBoxInst *ABI) {
       // instruction we found that isn't an explicit dealloc_box.
       Builder.emitDestroyAddrAndFold(Loc, valueToDestroy);
     }
+    auto *dbi = dyn_cast<DeallocBoxInst>(LastRelease);
+    if (dbi && dbi->isDeadEnd()) {
+      // Don't bother to create dealloc_stack instructions in dead-ends.
+      continue;
+    }
     Builder.createDeallocStack(Loc, ASI);
   }
 

test/SILOptimizer/allocbox_to_stack_lifetime.sil

@@ -50,3 +50,24 @@ bb0(%0 : $Int):
   return %3 : $Int
 }
 
+// CHECK-LABEL: sil [ossa] @keep_dead_end : {{.*}} {
+// CHECK:         [[STACK:%[^,]+]] = alloc_stack
+// CHECK:         cond_br undef, [[DIE:bb[0-9]+]]
+// CHECK:       [[DIE]]:
+// CHECK-NEXT:    unreachable
+// CHECK-LABEL: } // end sil function 'keep_dead_end'
+sil [ossa] @keep_dead_end : $@convention(thin) () -> () {
+bb0:
+  %b = alloc_box ${ var Int }
+  cond_br undef, die, exit
+
+die:
+  dealloc_box [dead_end] %b : ${ var Int }
+  unreachable
+
+exit:
+  dealloc_box %b : ${ var Int }
+  %retval = tuple ()
+  return %retval : $()
+
+}