[MachineOutliner][AArch64] Add support for saving LR to a register
This teaches the outliner to save LR to a register rather than the stack when
possible. This allows us to avoid bumping the stack in outlined functions in
some cases. By doing this, in a later patch, we can teach the outliner to do
something like this:
... bl OUTLINED_FUNCTION ...
... move LR's contents to a register bl OUTLINED_FUNCTION move the register's contents back
instead of falling back to saving LR in both cases.