[MachineOutliner][AArch64] Save + restore LR in noreturn functions

Conservatively always save + restore LR in noreturn functions.

These functions do not end in a RET, and so they aren't guaranteed to have an
instruction which uses LR in any way. So, as a result, you can end up in
unfortunate situations where you can't backtrace out of these functions in a
debugger.

Remove the old noreturn test, and add a new one which is more descriptive.

Remove the restriction that we can't outline from noreturn functions as well
since we now do the right thing.