[PPC CodeGen] Fix the bitreverse.i64 intrinsic.

Summary: The two 32-bit words were swapped. Update a test omitted in reverted r316270.

Reviewers: jtony, aaron.ballman

Subscribers: nemanjai, kbarton

Differential Revision: https://reviews.llvm.org/D39163

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316916 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed