[PowerPC] Add vec_vsx_ld and vec_vsx_st intrinsics

This patch enables the vec_vsx_ld and vec_vsx_st intrinsics for
PowerPC, which provide programmer access to the lxvd2x, lxvw4x,
stxvd2x, and stxvw4x instructions.

New code in altivec.h defines these in terms of new builtins, which
are themselves defined in BuiltinsPPC.def.  The builtins are converted
to LLVM intrinsics in CGBuiltin.cpp.  Additional code is added to
builtins-ppc-vsx.c to verify the correct generation of the intrinsics.

Note that I moved the other VSX builtins so all VSX builtins will be
alphabetical in their own section in BuiltinsPPC.def.

There is a companion patch for LLVM.

llvm-svn: 221768