nir: add lowering for idiv/udiv/umod

nir: add lowering for idiv/udiv/umod

Based on the algo from NV50LegalizeSSA::handleDIV() and handleMOD().
See also trans_idiv() in freedreno/ir3/ir3_compiler.c (which was an
adaptation of the nv50 code from Ilia Mirkin).

A python/numpy script which implements the same algorithm (and is
possibly useful for debugging or analysis) can be found here:

  http://people.freedesktop.org/~robclark/div-lowering.py

I've tested this on i965 hacked up to insert the idiv lowering pass,
and on freedreno with NIR frontend.

Signed-off-by: Rob Clark <robclark@freedesktop.org>
Tested-by: Eric Anholt <eric@anholt.net> (vc4)