[Tablegen][SubtargetEmitter] refactor method `emitSchedModelHelpersImpl()`. NFCI

Part of the logic has been moved to helper functions to (hopefully) improve
readability.
Added a few code comments to better describe how the algorithm works.

No functional change intended.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@339421 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed