In the machine outliner implementation for AArch64, signOutlinedFunction()
reimplements signing the LR value in prologue and authenticating it in
epilogue of the outlined function. This patch factors out signLR() and
authenticateLR() functions from AArch64FrameLowering code and reuses
them in signOutlinedFunction().
The mergeOutliningCandidateAttributes() outliner callback is
introduced as well to further unify signing and authentication of the LR
value.