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/CGExprScalar.cpp b/clang/lib/CodeGen/CGExprScalar.cpp
index 4338d1c..31487ac 100644
--- a/clang/lib/CodeGen/CGExprScalar.cpp
+++ b/clang/lib/CodeGen/CGExprScalar.cpp
@@ -3835,8 +3835,7 @@
             *SecondVecArg = RHS;
 
       QualType ElTy = LHSTy->castAs<VectorType>()->getElementType();
-      const BuiltinType *BTy = ElTy->getAs<BuiltinType>();
-      BuiltinType::Kind ElementKind = BTy->getKind();
+      BuiltinType::Kind ElementKind = ElTy->castAs<BuiltinType>()->getKind();
 
       switch(E->getOpcode()) {
       default: llvm_unreachable("is not a comparison operation");