div_qr_2

[Niels Möller]

I've now added a 4/2 division loop, based on earlier work of Torbjörn's.
It generates two quotient limbs at a time, while the old 3/2 loop
generates one quotient limb at a time.

Tuned via DIV_QR_2_PI2_THRESHOLD, which by default is infinity. I'm
actually not sure if the 4/2 C code (generating two quotient limbs at a
time) can beat the 3/2 division anywhere, but at least Torbjörns said
that an assembler implementation can be competitive.

But I added tuneup code (with check_size=500, so if it not better there,
the threshold is set to infinity). It will be interesting to see the
results for the nightly build.

Regards,
/Niels

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