As near as I can tell, other MMX instructions do not cause this
   problem -- instructions such as 'pxor' and 'por' are executed
   prior to the 'pand' in the rc5des code and they do not trigger
   this problem.  For some strange reason 'pand' bombs out as shown
   above.

Looks like a typo in the instruction definitions.

/usr/src/gnu/dist/include/opcode/i386.h defines it as:

{"pand",      2, 0x0fda, _, Modrm, { RegMMX|WordMem, RegMMX, 0 } },

while intel's web page on the MMX instructions would seem to imply the
correct opcode for pand starts with 0x0fdB.

(from http://developer.intel.com/drg/mmx/Manuals/prm/PRM_CHP5.HTM:

PAND-Bitwise Logical And	
	     Opcode		Instruction		Description
	     0F DB /r		PAND mm, mm/m64		AND 64 bits from MMX register/memory to MMX register. )

I leave it to the toolchain gurus to fix this..

					- Bill