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

[Peter Collingbourne via llvm-commits]

pcc wrote:

I changed my mind about https://github.com/llvm/llvm-project/pull/138366#issuecomment-2855889424 and decided that the LLVM assembler is correct and it's a linker bug that this case isn't handled.

I also updated this PR with two more bug fixes found during testing:
- Don't follow relocations to STT_GNU_IFUNC symbols
- Don't get stuck in an infinite loop when encountering a branch loop

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