[llvm] [BOLT][AArch64] Add call relaxation pass (PR #173952)

[Alexandros Lamprineas via llvm-commits]

labrinea wrote:

@rafaelauler thanks for clarifying. Can we perhaps write down these assumptions in a comment? I had a look at the patch and overall looks ok. Meanwhile I have been trying to relax unconditional branches left from relaxLocalBranches using a similar idea. I am creating fragment clusters and insert thunk chains for remote branches across such clusters. Perhaps I could push that for review, and/or raise an RFC. The aim is to support large binaries, but it's not tested in production workloads. It is expected to perform worse that non compact code model, but tries to move away from fragile layout guessing ahead of emission.

https://github.com/llvm/llvm-project/pull/173952