Fix for a backpatching bug reported by Allen.

  Because the boundary between short and long span-dependent
  instructions is +/- 128, there are an astounding number of
  span-dependent instructions whose size is over estimated.

  Allen came up with the idea of letting the size of span
  dependent instructions be non-monotonic, for a maxIter
  number of times, after which the size must be monotonically
  increasing.

  This table shows the number of span-dependent instructions
  whose size was over-estimated as a function of maxIter, for the
  file Parse/parse/ml.grm.sml:

     maxIter		# of instructions:
 	10			687
 	20			438
 	30			198
        40			  0

  In compiling the compiler, there is no significant difference in
  compilation speed between maxIter=10 and maxIter=40. Actually,
  my measurements showed that maxIter=40 was a tad faster than
  maxIter=10! Also 96% of the  files in the compiler reach a fix
  point within 13 iterations, so fixing maxIter at 40, while high,
  is okay.