[PowerPC] Implement vec_xxsldwi builtin.

The vec_xxsldwi builtin is missing from altivec.h. This has been requested by
developers working on libvpx for VP9 support for Google.

The patch fixes PR: https://bugs.llvm.org/show_bug.cgi?id=32653
Differential Revision: https://reviews.llvm.org/D33236

llvm-svn: 303766