Fix "pointer is null" static analyzer warnings. NFCI.

Use castAs<> instead of getAs<> since the pointer is dereferenced immediately below and castAs will perform the null assertion for us.
diff --git a/clang/lib/CodeGen/CGObjC.cpp b/clang/lib/CodeGen/CGObjC.cpp
index 3c11aa7..c52aa6b 100644
--- a/clang/lib/CodeGen/CGObjC.cpp
+++ b/clang/lib/CodeGen/CGObjC.cpp
@@ -488,7 +488,7 @@
     Receiver = CGF.EmitScalarExpr(SelfInClassMethod);
   } else {
     QualType ReceiverType = SubOME->getClassReceiver();
-    const ObjCObjectType *ObjTy = ReceiverType->getAs<ObjCObjectType>();
+    const ObjCObjectType *ObjTy = ReceiverType->castAs<ObjCObjectType>();
     const ObjCInterfaceDecl *ID = ObjTy->getInterface();
     assert(ID && "null interface should be impossible here");
     Receiver = CGF.CGM.getObjCRuntime().GetClass(CGF, ID);
@@ -555,9 +555,7 @@
 
   case ObjCMessageExpr::Class: {
     ReceiverType = E->getClassReceiver();
-    const ObjCObjectType *ObjTy = ReceiverType->getAs<ObjCObjectType>();
-    assert(ObjTy && "Invalid Objective-C class message send");
-    OID = ObjTy->getInterface();
+    OID = ReceiverType->castAs<ObjCObjectType>()->getInterface();
     assert(OID && "Invalid Objective-C class message send");
     Receiver = Runtime.GetClass(*this, OID);
     isClassMessage = true;