[PPC] support for arithmetic builtins in the FE

(commit again after fixing the buildbot failures)
This adds various overloads of the following builtins to altivec.h:

    vec_neg
    vec_nabs
    vec_adde
    vec_addec
    vec_sube
    vec_subec
    vec_subc

Note that for vec_sub builtins on 32 bit integers, the semantics is similar to
what ISA describes for instructions like vsubecuq that work on quadwords: the
first operand is added to the one's complement of the second operand. (As
opposed to two's complement which I expected).

llvm-svn: 287872