Mark a test as requiring a default triple

This test assumes that llc can infer a default triple. I'm not sure why
exactly, but the Verify MachineInstrs bot requires tests to be explicit
about this dependency.

This commit follows the lead from r248452 and adds in 'REQUIRES:
default_triple' to omit-empty.ll.

Bot URL: http://lab.llvm.org:8080/green/job/Verify-Machineinstrs_AArch64/7500

llvm-svn: 304269