Use builtin_widen_mult_even/odd in expand_vector_divmod

[Richard Henderson <rth at redhat dot com>]

We use it everywhere else, but it got forgotten here.  Saves two shuffles on Altivec.

Although with all of this duplication it makes me wonder if we shouldn't just give up
on the idea of auto-generating MULT_HIGHPART from other operations (particularly given
the extra type frobbing involved).  In some sense it'd be easier to just add a couple
of lines to the backends to implement the operation and be done with it.  Thoughts?

That said, tested on ppc64-linux.  Committed.


r~

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