[Tablegen][SubtargetEmitter] refactor method `emitSchedModelHelpersImpl()`. NFCI
[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: 0ff597fd157e6f4fc38580e8d64ab130330d2411@339421 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm