[XRay] Fix XRay PPC return value bug.

Summary:
This bug is caused by the incorrect handling of return-value registers.

According to OpenPOWER 64-Bit ELF V2 ABI 2.2.5, up to 2 general-purpose
registers are going to be used for return values, and up to 8 floating
point registers or vector registers are going to be used for return
values.

Reviewers: dberris, echristo

Subscribers: nemanjai, llvm-commits

Differential Revision: https://reviews.llvm.org/D33027

llvm-svn: 302691