Change sample profile writer to make it deterministic.

Summary: This patch changes the function profile output order to be deterministic. In order to make it easier to understand, hottest functions (with most total samples) is ordered first.

Reviewers: dnovillo, davidxl

Reviewed By: dnovillo

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D33111

llvm-svn: 302851