[SSADestroyHoisting] Fixed barrier check.

Previously, FindBarrierAccessScopes::checkReachablePhiBarrier was not
looking at the terminator of predecessors but rather looking at the
terminator of block itself.  Previously, in cases where the current
block's terminator was in fact a barrier, that resulted in failing to
hoist any live-in access scopes.

Now that we aren't running the data flow twice, the result was worse: in
cases where the current block's terminator was a barrier but there was
no access scope in play, no barrier would be added at all.

Author: nate-chandler

lib/SILOptimizer/Transforms/SSADestroyHoisting.cpp

@@ -367,7 +367,7 @@ class DeinitBarriers {
     bool checkReachablePhiBarrier(SILBasicBlock *block) {
       bool isBarrier =
           llvm::any_of(block->getPredecessorBlocks(), [&](auto *predecessor) {
-            return result.isBarrier(block->getTerminator());
+            return result.isBarrier(predecessor->getTerminator());
           });
       if (isBarrier) {
         // If there's a barrier preventing us from hoisting out of this block,

test/SILOptimizer/hoist_destroy_addr.sil

@@ -43,12 +43,22 @@ struct TrivialStruct {
   var e: E
 }
 
+enum Change {
+case insert(offset: Int, element: X)
+case remove(offset: Int, element: X)
+}
+
+struct Int {
+    @_hasStorage var _value : Builtin.Int64
+}
+
 sil @unknown : $@convention(thin) () -> ()
 sil @use_S : $@convention(thin) (@in_guaranteed S) -> ()
 
 sil @f_out : $@convention(thin) <T> () -> @out T
 sil @f_bool : $@convention(thin) () -> Builtin.Int1
 sil [ossa] @take_trivial_struct : $@convention(thin) (TrivialStruct) -> ()
+sil [ossa] @get_change_out : $@convention(thin) () -> @out Change
 
 // CHECK-LABEL: sil [ossa] @test_simple
 // CHECK:      bb0(%0 : $*S):
@@ -270,6 +280,53 @@ entry(%addr : $*X):
   return %tuple : $()
 }
 
+// Hoist a destroy_addr over a phi.
+//
+// CHECK-LABEL: sil [ossa] @hoist_over_undef_phi : {{.*}} {
+// CHECK:         [[STACK1:%[^,]+]] = alloc_stack
+// CHECK-NEXT:    apply undef
+// CHECK-NEXT:    destroy_addr [[STACK1]]
+// CHECK-LABEL: } // end sil function 'hoist_over_undef_phi'
+sil [ossa] @hoist_over_undef_phi : $@convention(thin) () -> () {
+entry:
+  br latch
+
+latch:
+  cond_br undef, top, exit
+
+top:
+  %stack1 = alloc_stack $Change
+  apply undef(%stack1) : $@convention(thin) () -> @out Change 
+  %access = begin_access [static] [modify] %stack1 : $*Change
+  br top2(undef : $())
+
+top2(%66 : $()):
+  end_access %access : $*Change
+  destroy_addr %stack1 : $*Change
+  dealloc_stack %stack1 : $*Change
+  %stack2 = alloc_stack $Change
+  apply undef(%stack2) : $@convention(thin) () -> @out Change 
+  switch_enum_addr %stack2 : $*Change, case #Change.insert!enumelt: left, case #Change.remove!enumelt: right
+
+left:
+  br bottom
+
+right:
+  br bottom
+
+bottom:
+  destroy_addr %stack2 : $*Change
+  dealloc_stack %stack2 : $*Change
+  br backedge
+
+backedge:
+  br latch
+
+exit:
+  %321 = tuple ()
+  return %321 : $()
+}
+
 // Fold destroy_addr and a load [copy] into a load [take] even when that
 // load [take] is guarded by an access scope.
 //