[Bridging cast optimization] Unchecked casts can perform a class downcast.

Handle a class downcast following a bridging cast. Fixes
rdar://problem/31791421.

Author: DougGregor

lib/SILOptimizer/Utils/Local.cpp

@@ -1661,6 +1661,9 @@ optimizeBridgedSwiftToObjCCast(SILInstruction *Inst,
     if ((ConvTy == DestTy || DestTy.isExactSuperclassOf(ConvTy))) {
       CastedValue = SILValue(
           (ConvTy == DestTy) ? NewI : Builder.createUpcast(Loc, NewAI, DestTy));
+    } else if (ConvTy.isExactSuperclassOf(DestTy)) {
+      CastedValue = SILValue(
+          Builder.createUnconditionalCheckedCast(Loc, NewAI, DestTy));
     } else if (ConvTy.getSwiftRValueType() ==
                    getNSBridgedClassOfCFClass(M.getSwiftModule(),
                                               DestTy.getSwiftRValueType()) ||
@@ -1673,8 +1676,7 @@ optimizeBridgedSwiftToObjCCast(SILInstruction *Inst,
     } else {
       llvm_unreachable(
           "Destination should have the same type, be bridgeable CF "
-          "type or be a superclass "
-          "of the source operand");
+          "type or be a superclass/subclass of the source operand");
     }
     NewI = Builder.createStore(Loc, CastedValue, Dest,
                                StoreOwnershipQualifier::Unqualified);

test/SILOptimizer/bridged_casts_folding.swift

@@ -900,4 +900,16 @@ public func testCondCastSwiftToCFSetInt() -> CFSet? {
   return setOpt
 }
 
+public class NSObjectSubclass : NSObject { }
 
+var anyHashable: AnyHashable = 0
+
+// CHECK-LABEL: _T021bridged_casts_folding29testUncondCastSwiftToSubclassAA08NSObjectI0CyF
+// CHECK: [[GLOBAL:%[0-9]+]] = global_addr @_T021bridged_casts_folding11anyHashables03AnyE0Vv
+// CHECK: function_ref @_T0s11AnyHashableV10FoundationE19_bridgeToObjectiveCSo8NSObjectCyF
+// CHECK-NEXT: apply
+// CHECK-NEXT: unconditional_checked_cast {{%.*}} : $NSObject to $NSObjectSubclass
+@inline(never)
+public func testUncondCastSwiftToSubclass() -> NSObjectSubclass {
+  return anyHashable as! NSObjectSubclass
+}