Cranelift AArch64: Fix the get_return_address lowering

[akirilov-arm]

The previous implementation assumed that nothing had clobbered the LR register since the current function had started executing, so it would be incorrect for a non-leaf function, for example, that contains the get_return_address operation right after a call. The operation is valid only if the preserve_frame_pointers flag is enabled, which implies that the presence of a frame record on the stack is guaranteed.

[cfallin]

Thanks! One request for an additional comment below.

At the top level, so I understand correctly: the verifier already ensures that this op can only be used when preserve_frame_pointers is true; so the additional conditions here are to document that / ensure it remains the case?

If so, as another extra assurance, it may be good to put a comment by the verifier check (linked above) indicating that backends may rely on this to be true and the check should not be loosened without auditing all backends.

Thanks!

[akirilov-arm]

At the top level, so I understand correctly: the verifier already ensures that this op can only be used when preserve_frame_pointers is true; so the additional conditions here are to document that / ensure it remains the case?

Yes, I added the check as an additional safety net, and your suggestion to write a comment next to the verifier check makes perfect sense, thanks!