Use isa-masking to read the class object pointer from

instances of Swift subclasses of ObjC classes.

We were already doing this in the runtime.  This patch
unhides the runtime's mask word (swift_isaMask) and makes
IR-gen take advantage of it when it can.

Swift SVN r21592

lib/IRGen/IRGenModule.cpp

@@ -380,6 +380,16 @@ llvm::Constant *IRGenModule::getObjCEmptyVTablePtr() {
   return ObjCEmptyVTablePtr;
 }
 
+Address IRGenModule::getAddrOfObjCISAMask() {
+  // This symbol is only exported by the runtime if the platform uses
+  // isa masking.
+  assert(TargetInfo.hasISAMasking());
+  if (!ObjCISAMaskPtr) {
+    ObjCISAMaskPtr =  Module.getOrInsertGlobal("swift_isaMask", IntPtrTy);
+  }
+  return Address(ObjCISAMaskPtr, getPointerAlignment());
+}
+
 llvm::Module *IRGenModule::getModule() const {
   return ClangCodeGen->GetModule();
 }