[llvm-branch-commits] [BOLT][BAT] Add support for three-way split functions (PR #93760)

[Amir Ayupov via llvm-branch-commits]

https://github.com/aaupov created https://github.com/llvm/llvm-project/pull/93760

In three-way split functions, if only .warm fragment is present, BAT
incorrectly overwrites the map for .warm fragment by empty .cold
fragment.

Test Plan: updated register-fragments-bolt-symbols.s