[llvm-branch-commits] [BOLT][BAT] Support multi-way split functions (PR #87123)

[Amir Ayupov via llvm-branch-commits]

https://github.com/aaupov created https://github.com/llvm/llvm-project/pull/87123

BAT writeMaps encoded the assumption that functions are only split into
two fragments (hot and cold). However, BOLT supports splitting into
arbitrary number of fragments. Relax that assumption and look up primary
(hot) fragment explicitly.

Test Plan: Updated bolt/test/X86/yaml-secondary-entry-discriminator.s