[PowerPC] Add VSX builtins for vec_div

This patch adds builtin support for xvdivdp and xvdivsp, along with a
new test case.  The builtins are accessed using vec_div in altivec.h.
Builtins are listed (mostly) alphabetically there, so inserting these
changed the line numbers for deprecation warnings tested in
test/Headers/altivec-intrin.c.

There is a companion patch for LLVM.

llvm-svn: 221984