SimplifyCFG: Refactor GatherConstantCompares() result in a struct

Code seems cleaner and easier to understand this way

This is basically r222416, after fixes for MSVC lack of standard 
support, and a few cleaning (got rid of a warning).
Thanks Nakamura Takumi and Nico Weber for the MSVC fixes.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@222472 91177308-0d34-0410-b5e6-96231b3b80d8