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.
Depends on: https://github.com/llvm/llvm-project/pull/86219
Test Plan: Updated bolt/test/X86/yaml-secondary-entry-discriminator.s
Reviewers: ayermolo, rafaelauler, maksfb, dcci
Reviewed By: maksfb, dcci
Pull Request: https://github.com/llvm/llvm-project/pull/87123
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