[NFC][llvm-exegesis] Refactor ResolvedSchedClass & friends

Summary:
`ResolvedSchedClass` will need to be used outside of `Analysis`
(before `InstructionBenchmarkClustering` even), therefore promote
it into a non-private top-level class, and while there also
move all of the functions that are only called by `ResolvedSchedClass`
into that same new file.

Reviewers: courbet, gchatelet

Reviewed By: courbet

Subscribers: mgorny, tschuett, mgrang, jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D59993

llvm-svn: 357259