[AArch64] Robustize neon-scalar-copy.ll tests. NFC.

Some of those didn't even have run lines: they were removed
inadvertently during the Great Merge of 2014.

They used to check for DUPs, but now we go through W-regs?
Filed PR22418 for that potential regression.

For now, just make the tests explicit, so we now where we stand.

llvm-svn: 227635