deflate_compress: clean up deflate_compress_{greedy,lazy,near_optimal}()
deflate_compress: clean up deflate_compress_{greedy,lazy,near_optimal}()

Various minor cleanups, such as adjusting the coding style and
refactoring some logic into a helper function.  No "real" changes.

ebiggers/libdeflate,ebiggers/libdeflate,ebiggers/libdeflate