Return nullopt if Reg is undef. (#155893)
In describeORRLoadedValue in AArch64InstrInfo.cpp, we try to check if an
instruction is a copy like instruction, the isCopyLikeInstr function
returns a pair of destination and source registers. If any of them are
undef, we should just return a nullopt to avoid any crashes later in the
code when trying to get the SubReg for one of those registers.
rdar://158581204
