x86 update -- raytrace benchmark

[Stephen Weeks]

> So, raytrace.sml works with both -DMLton_safe=1 and -DMLton_safe=0.  I'm
> including the updated allocate-registers.fun.  I'm also tracking down
> another minor bug, but I think it was introduced by some changes I made to
> the translation phase.  Let me know if you encounter anything odd with
> this version.

It worked like a charm.  Here are the new raytrace benchmark numbers.  Quite
good!

                    MLton   MLton
              OCAML     C  native  SML/NJ
holes           1.8   3.5     3.0     3.9
fov             1.5   1.8     1.6     3.2
intercyl        1.6   2.0     1.8     4.3
snowgoon        2.9   3.2     2.6     5.1
dice            3.9   5.1     4.2     8.8
golf            1.5   2.5     2.1     3.1
cone-fractal    3.7   4.4     3.4     6.5
large           4.3   3.5     2.8     6.7
pipe            5.4   4.7     4.0     7.9
chess          16.0  15.9    12.3    21.6
fractal        12.2  13.8    10.8    45.4
geom-mean       3.6   4.3     3.5     7.1