[AArch64] Fix unintended fallthrough and strengthen cast

This was added in r330630. GCC's -Wimplicit-fallthrough seems to not
fire when the previous case contains a switch itself.

This fallthrough was bening because the helper function implementing the
case used dyn_cast to re-check the type of the node in question. After
fixing the fallthrough, we can strengthen the cast.

llvm-svn: 345864