[XRay] fix 99th percentile lookups by sorting the array correctly

Summary:
It was a copy-paste typo, sorting only to the 90th percentile twice.
Now, it only sorts the array prefix once, and extracts what we need.

Reviewers: dberris, kpw, eizan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D42690

llvm-svn: 323800