x = x/R mod m (HAC 14.32)
function montReduce(x) {
  var x_array = x.array;
  while(x.t <= this.mt2)	// pad x so am has enough room later
    x_array[x.t++] = 0;
  for(var i = 0; i < this.m.t; ++i) {
    // faster way of calculating u0 = x[i]*mp mod DV
    var j = x_array[i]&0x7fff;
    var u0 = (j*this.mpl+(((j*this.mph+(x_array[i]>>15)*this.mpl)&this.um)<<15))&BI_DM;
    // use am to combine the multiply-shift-add into one call
    j = i+this.m.t;
    x_array[j] += this.m.am(0,u0,x,i,0,this.m.t);
    // propagate carry
    while(x_array[j] >= BI_DV) { x_array[j] -= BI_DV; x_array[++j]++; }
  }
  x.clamp();
  x.drShiftTo(this.m.t,x);
  if(x.compareTo(this.m) >= 0) x.subTo(this.m,x);
}