[lld] ELF: Add branch-to-branch optimization. (PR #138366)

[Peter Smith via llvm-commits]

https://github.com/smithp35 commented:

I've made a few suggestions for some names that I found a bit confusing. The AArch64 side looks good to me. I've not checked the x86_64 side.

https://github.com/llvm/llvm-project/pull/138366