[PowerPC] Use xxleqv to set all one vector IMM(-1).

Summary:
xxspltib/vspltisb are 3 cycle PM instructions,
xxleqv is 2 cycle ALU instruction.

We should use xxleqv to set all one vectors.

Reviewers: hfinkel, nemanjai, steven.zhang

Subscribers: hiraditya, kbarton, MaskRay, shchenz, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65529

llvm-svn: 369006