Codegen - silence static analyzer getAs<> null dereference warnings. NFCI.

The static analyzer is warning about potential null dereferences, but in these cases we should be able to use castAs<> directly and if not assert will fire for us.

llvm-svn: 373918
diff --git a/clang/lib/CodeGen/CGAtomic.cpp b/clang/lib/CodeGen/CGAtomic.cpp
index afddfb7..5059163 100644
--- a/clang/lib/CodeGen/CGAtomic.cpp
+++ b/clang/lib/CodeGen/CGAtomic.cpp
@@ -970,7 +970,7 @@
     auto CastToGenericAddrSpace = [&](llvm::Value *V, QualType PT) {
       if (!E->isOpenCL())
         return V;
-      auto AS = PT->getAs<PointerType>()->getPointeeType().getAddressSpace();
+      auto AS = PT->castAs<PointerType>()->getPointeeType().getAddressSpace();
       if (AS == LangAS::opencl_generic)
         return V;
       auto DestAS = getContext().getTargetAddressSpace(LangAS::opencl_generic);