[NFC] Variable auto-init: use getAsVariableArrayType helper

As suggested by @rjmccall in D57797.

llvm-svn: 353490
diff --git a/clang/lib/CodeGen/CGDecl.cpp b/clang/lib/CodeGen/CGDecl.cpp
index 2510344..f91fb01 100644
--- a/clang/lib/CodeGen/CGDecl.cpp
+++ b/clang/lib/CodeGen/CGDecl.cpp
@@ -1658,8 +1658,7 @@
     // Technically zero-sized or negative-sized VLAs are undefined, and UBSan
     // will catch that code, but there exists code which generates zero-sized
     // VLAs. Be nice and initialize whatever they requested.
-    const VariableArrayType *VlaType =
-        dyn_cast_or_null<VariableArrayType>(getContext().getAsArrayType(type));
+    const auto *VlaType = getContext().getAsVariableArrayType(type);
     if (!VlaType)
       return;
     auto VlaSize = getVLASize(VlaType);