This patch adds support for the vector merge even word and vector merge odd word

instructions introduced in POWER8.

These are the Clang-related changes for http://reviews.llvm.org/D10704

All builtins are added in altivec.h and guarded with the POWER8_VECTOR macro.

Phabricator review: http://reviews.llvm.org/D10736

llvm-svn: 241293