[llvm-exegesis] Fix function return generation so it doesn't return register 0

When fillMachineFunction generates a return on targets without a return opcode
(such as AArch64) it should pass an empty set of registers as the return
registers, not 0 which means register number zero.

Differential Revision: https://reviews.llvm.org/D53074


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@344139 91177308-0d34-0410-b5e6-96231b3b80d8