[PowerPC] Initial VSX intrinsic support, with min/max for vector double

Now that we have initial support for VSX, we can begin adding
intrinsics for programmer access to VSX instructions.  This patch adds
basic support for VSX intrinsics in general, and tests it by
implementing intrinsics for minimum and maximum for the vector double
data type.

The LLVM portion of this is quite straightforward.  There is a
companion patch for Clang.

llvm-svn: 220988