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.

llvm-svn: 222472
1 file changed