[FastISel][AArch64] Allow handling of vectors during return lowering for little endian machines.

Allow handling of vectors during return lowering at least for little endian machines.
This was restricted in r208200 to fix it for big endian machines (according to
the comment), but it also disabled it for little endian too.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@217846 91177308-0d34-0410-b5e6-96231b3b80d8