[AArch64] Move peephole-sxtw.mir file to regression test directory (l…

…lvm#100819)

In llvm#96293 ("Remove superfluous sxtw in peephole opt") a file
`peephole-sxtw.mir` was added to `llvm/lib/Target/AArch64/`, but it
looks like this is a regression test file.

Move it to `llvm/test/CodeGen/AArch64/` which seems a more correct
location.