In an earlier commit 584d0d5c1749c13625a5d322178ccb4121eea610 I
added functionality to allow AArch64 CodeGen support for falling
back to DAG ISel when Global ISel encounters scalable vector
types. However, it seems that we were not falling back early
enough as llvm::getLLTForType was still being invoked for scalable
I've added a new fallback function to the call lowering class in
order to catch this problem early enough, rather than wait for
lowerFormalArguments to reject scalable vector types.