CastOptimizer: fix a small deficiency when optimizing checked_cast_addr_br instructions.

Handle destroy_addr of the destination. When replacing the cast, just remove such destroys.
In real world SIL there will always be a destroy_addr in the success-branch of the cast.
Not sure if this optimization could have ever kicked in for real world code.

Author: eeckstein

lib/SILOptimizer/Utils/CastOptimizer.cpp

@@ -873,7 +873,8 @@ SILInstruction *CastOptimizer::simplifyCheckedCastAddrBranchInst(
   if (ResultNotUsed) {
     for (auto Use : Dest->getUses()) {
       auto *User = Use->getUser();
-      if (isa<DeallocStackInst>(User) || User == Inst)
+      if (isa<DeallocStackInst>(User) || isa<DestroyAddrInst>(User) ||
+          User == Inst)
         continue;
       ResultNotUsed = false;
       break;
@@ -906,6 +907,11 @@ SILInstruction *CastOptimizer::simplifyCheckedCastAddrBranchInst(
         auto &srcTL = Builder.getTypeLowering(Src->getType());
         srcTL.emitDestroyAddress(Builder, Loc, Src);
       }
+      for (auto iter = Dest->use_begin(); iter != Dest->use_end();) {
+        SILInstruction *user = (*iter++)->getUser();
+        if (isa<DestroyAddrInst>(user))
+          eraseInstAction(user);
+      }
       eraseInstAction(Inst);
       Builder.setInsertionPoint(BB);
       auto *NewI = Builder.createBranch(Loc, SuccessBB);

test/SILOptimizer/simplify_cfg_ossa.sil

@@ -1549,6 +1549,7 @@ bb0(%0 : $*IsQ):
 
 bb1:
   %m1 = integer_literal $Builtin.Int1, -1
+  destroy_addr %p : $*Q
   br bb2(%m1 : $Builtin.Int1)
 
 bb2(%5 : $Builtin.Int1):