large compile times for mlyacc

[Stephen Weeks]

Something definitely needs to get done.  Here is a self compile that I
just ran.

MLton starting
   Compile SML starting
      pre codegen starting
	 gcc -DNODEBUG -DMLton_safe=TRUE -DExn_keepHistory=FALSE \
	     -DMLton_detectOverflow=TRUE -I/home/sweeks/mlton/include -o \
	     /tmp/fileSjEDyD /tmp/fileOvuRkz.c -L/home/sweeks/mlton/lib \
	     -lmlton -lm -lgmp
	 /tmp/fileSjEDyD /tmp/filewp23g1
      pre codegen finished in 112.92 + 127.35 (53% GC)
      x86 code gen starting
      x86 code gen finished in 576.44 + 175.74 (23% GC)
   Compile SML finished in 689.36 + 303.09 (31% GC)

Everything looks fine except for the x86codegen mutator time, which is
>2X what it usually is.