[XRay] support conditional return on PPC.

Summary: Conditional returns were not taken into consideration at all. Implement them by turning them into jumps and normal returns. This means there is a slightly higher performance penalty for conditional returns, but this is the best we can do, and it still disturbs little of the rest.

Reviewers: dberris, echristo

Subscribers: sanjoy, nemanjai, hiraditya, kbarton, llvm-commits

Differential Revision: https://reviews.llvm.org/D38102

llvm-svn: 314005