Use isFunctionHotInCallGraph to set the function section prefix.

Summary: The current prefix based function layout algorithm only looks at function's entry count, which is not sufficient. A function should be grouped together if its entry count or any call edge count is hot.

Reviewers: davidxl, eraman

Reviewed By: eraman

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31225

llvm-svn: 298656