[BOLT] Fix sorting functions by execution count

I noticed that `-reorder-functions=exec-count` doesn't work as expected due to
a bug in the comparison function (which isn't symmetric). It is questionable
whether anyone would want to ever use the sorting method (as sorting by say
density is much better in all cases) but it is probably better to fix the bug.

Reviewed By: Amir

Differential Revision: https://reviews.llvm.org/D152959