Fixed a crasher that could happen if:

- you have a type that contains a typedef to a VectorType or an ExtVectorType
- that type is returned from an ARM function that LLDB steps over so we try to figure out the return type
- we try to determine if the type is a homogeneous aggregate type and we crash

We get not using getAs() when we should have been and using llvm::cast caused an assertion crash when the typedef type didn't return a valid VectorType or ExtVectorType.

<rdar://problem/19646550>

llvm-svn: 228771