[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:

f1:
  ...
  bl OUTLINED_FUNCTION
  ...

f2:
  ...
  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.

llvm-svn: 338278