Fix prefix order for "3-byte" instructions like the SIMD instructions.

Fix prefix order for "3-byte" instructions like the SIMD instructions.
While the F2/F3/66 prefix used by these instructions should precede the
REX prefix in 64-bit mode, they should probably not precede the segment
override.  The processor specifications seem to be unclear on this point,
so play it safe.

This change has a minor side-effect of reversing the segreg/size override
ordering on things like "fs cmpsd" in 16-bit mode; this is okay as those
prefixes can be in any order.  This is due to the way Yasm implements
segreg overrides for instructions with no EA.

Reported by: Danny Reeves <danny_reeves@hotmail.com>

svn path=/trunk/yasm/; revision=1666